fix authorized_keys in check_mode

This change is in response to issue #1515.
Original pull request #1580.

The original problem is: in authorized_key module you have no idea about users
which will be created by Ansible at first run. I can propose next two ways to
solve this problem:

  1. Combine modules system/user.py and system/authorized_key.py in one module
     (so you will know everything about users in that module)
  2. Use small workaround: add my commit and always provide 'path' parameter
     for authorized_key module during runs with --check option.

system/authorized_key.py

@@ -168,6 +168,13 @@ def keyfile(module, user, write=False, path=None, manage_dir=True):
     :return: full path string to authorized_keys for user
     """
 
+    if module.check_mode:
+        if path is None:
+            module.fail_json(msg="You must provide full path to key file in check mode")
+        else:
+            keysfile = path
+            return keysfile
+
     try:
         user_entry = pwd.getpwnam(user)
     except KeyError, e: