Add a new unique_name param to the digital_ocean module for idempotence.

As it stands now, it is difficult to write idempotent tasks for digital
ocean droplets.  Digital ocean assigns new nodes a random id when they
are provisioned and that id is the only key that can be used to identify
it in subsequent runs of that play.

The workflow previously involved manual intervention:

- write a play defining a new node with no specified id
- run it, collect the randomly assigned id by hand
- modify the play to add the id by hand so future runs don't create
  duplicate nodes
- perform future re-runs that check if the node exists (by its id)
  - if it does exist then do nothing.
  - if it does not exist, then create it and return a *new random id*
  - collect the new random id by hand, modify the playbook file, and
    start all over.

Its a huge pain.

The modifications in this commit allow you to use the 'hostname' as a
primary key for idempotence with digital ocean.  By default, digital
ocean will let you create as many hosts with the same hostname as you
like.  Here, we provide an option to constrain the user to using only
unique hostnames.

The workflow will now look like:

- write a play defining a new node with a specified hostname and
  "unique_name: true""
- run it, create the new node and move on.
- re-run it, notice that a node with that hostname is already created
  and move on.

library/cloud/digital_ocean

@@ -45,6 +45,11 @@ options:
   name:
     description:
      - String, this is the name of the droplet - must be formatted by hostname rules, or the name of a SSH key.
+  unique_name:
+    description:
+     - Bool, require unique hostnames.  By default, digital ocean allows multiple hosts with the same name.  Setting this to "yes" allows only one host per name.  Useful for idempotence.
+    default: "no"
+    choices: [ "yes", "no" ]
   size_id:
     description:
      - Numeric, this is the id of the size you would like the droplet created at.
@@ -211,13 +216,22 @@ class Droplet(JsonfyMixIn):
         return droplet
 
     @classmethod
-    def find(cls, id):
+    def find(cls, id=None, name=None):
-        if not id:
+        if not id and not name:
             return False
+
         droplets = cls.list_all()
+
+        # Check first by id.  digital ocean requires that it be unique
         for droplet in droplets:
             if droplet.id == id:
                 return droplet
+
+        # Failing that, check by hostname.
+        for droplet in droplets:
+            if droplet.name == name:
+                return droplet
+
         return False
 
     @classmethod
@@ -281,7 +295,17 @@ def core(module):
     if command == 'droplet':
         Droplet.setup(client_id, api_key)
         if state in ('active', 'present'):
-            droplet = Droplet.find(module.params['id'])
+
+            # First, try to find a droplet by id.
+            droplet = Droplet.find(id=module.params['id'])
+
+            # If we couldn't find the droplet and the user is allowing unique
+            # hostnames, then check to see if a droplet with the specified
+            # hostname already exists.
+            if not droplet and module.params['unique_name']:
+                droplet = Droplet.find(name=getkeyordie('name'))
+
+            # If both of those attempts failed, then create a new droplet.
             if not droplet:
                 droplet = Droplet.add(
                     name=getkeyordie('name'),
@@ -290,18 +314,30 @@ def core(module):
                     region_id=getkeyordie('region_id'),
                     ssh_key_ids=module.params['ssh_key_ids']
                 )
+
             if droplet.is_powered_on():
                 changed = False
+
             droplet.ensure_powered_on(
                 wait=getkeyordie('wait'),
                 wait_timeout=getkeyordie('wait_timeout')
             )
+
             module.exit_json(changed=changed, droplet=droplet.to_json())
 
         elif state in ('absent', 'deleted'):
-            droplet = Droplet.find(getkeyordie('id'))
+            # First, try to find a droplet by id.
+            droplet = Droplet.find(id=getkeyordie('id'))
+
+            # If we couldn't find the droplet and the user is allowing unique
+            # hostnames, then check to see if a droplet with the specified
+            # hostname already exists.
+            if not droplet and module.params['unique_name']:
+                droplet = Droplet.find(name=getkeyordie('name'))
+
             if not droplet:
                 module.exit_json(changed=False, msg='The droplet is not found.')
+
             event_json = droplet.destroy()
             module.exit_json(changed=True, event_id=event_json['event_id'])
 
@@ -336,6 +372,7 @@ def main():
             region_id = dict(type='int'),
             ssh_key_ids = dict(default=''),
             id = dict(aliases=['droplet_id'], type='int'),
+            unique_name = dict(type='bool', choices=BOOLEANS, default='no'),
             wait = dict(type='bool', choices=BOOLEANS, default='yes'),
             wait_timeout = dict(default=300, type='int'),
             ssh_pub_key = dict(type='str'),