Add idempotency to mongodb_user module

database/mongodb_user

@@ -188,6 +188,18 @@ def main():
 
     try:
         client = MongoClient(login_host, int(login_port))
+    except ConnectionFailure, e:
+        module.fail_json(msg='unable to connect to database, check login_host and login_port are correct')
+
+    # try to authenticate as a target user to check if it already exists
+    try:
+        client[db_name].authenticate(user, password)
+        if state == 'present':
+            module.exit_json(changed=False, user=user)
+    except OperationFailure:
+        if state == 'absent':
+            module.exit_json(changed=False, user=user)
+
     if login_user is None and login_password is None:
         mongocnf_creds = load_mongocnf()
         if mongocnf_creds is not False:
@@ -199,9 +211,6 @@ def main():
     if login_user is not None and login_password is not None:
         client.admin.authenticate(login_user, login_password)
 
-    except ConnectionFailure, e:
-        module.fail_json(msg='unable to connect to database, check login_user and login_password are correct')
-
     if state == 'present':
         if password is None:
             module.fail_json(msg='password parameter required when adding a user')