Add executable parameter to find gem binary in gem module

This is especially needed with rvm where gem binary can be in unusual locations

packaging/gem

@@ -57,6 +57,11 @@ options:
     required: false
     default: "yes"
     version_added: "1.3"
+  executable:
+    description:
+    - Override the path to the gem executable
+    required: false
+    version_added: "1.4"
   version:
     description:
       - Version of the gem to be installed/removed.
@@ -77,8 +82,14 @@ EXAMPLES = '''
 
 import re
 
+def get_rubygems_path(module):
+    if module.params['executable']:
+        return module.params['executable']
+    else:
+        return module.get_bin_path('gem', True)
+
 def get_rubygems_version(module):
-    cmd = [module.get_bin_path('gem', True), '--version']
+    cmd = [get_rubygems_path(module), '--version']
     (rc, out, err) = module.run_command(cmd, check_rc=True)
 
     match = re.match(r'^(\d+)\.(\d+)\.(\d+)', out)
@@ -89,7 +100,7 @@ def get_rubygems_version(module):
 
 def get_installed_versions(module, remote=False):
 
-    cmd = [ module.get_bin_path('gem', True) ]
+    cmd = [ get_rubygems_path(module) ]
     cmd.append('query')
     if remote:
         cmd.append('--remote')
@@ -126,7 +137,7 @@ def uninstall(module):
 
     if module.check_mode:
         return
-    cmd = [ module.get_bin_path('gem', True) ]
+    cmd = [ get_rubygems_path(module) ]
     cmd.append('uninstall')
     if module.params['version']:
         cmd.extend([ '--version', module.params['version'] ])
@@ -146,7 +157,7 @@ def install(module):
     else:
         major = None
 
-    cmd = [ module.get_bin_path('gem', True) ]
+    cmd = [ get_rubygems_path(module) ]
     cmd.append('install')
     if module.params['version']:
         cmd.extend([ '--version', module.params['version'] ])
@@ -170,6 +181,7 @@ def main():
 
     module = AnsibleModule(
         argument_spec = dict(
+            executable           = dict(required=False, type='str'),
             gem_source           = dict(required=False, type='str'),
             include_dependencies = dict(required=False, default=True, type='bool'),
             name                 = dict(required=True, type='str'),