Add idempotency to mongodb_user module

database/mongodb_user

@@ -188,19 +188,28 @@ def main():
 
     try:
         client = MongoClient(login_host, int(login_port))
-        if login_user is None and login_password is None:
-            mongocnf_creds = load_mongocnf()
-            if mongocnf_creds is not False:
-                login_user = mongocnf_creds['user']
-                login_password = mongocnf_creds['password']
-        elif login_password is None and login_user is not None:
-            module.fail_json(msg='when supplying login arguments, both login_user and login_password must be provided')
-
-        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')
+        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:
+            login_user = mongocnf_creds['user']
+            login_password = mongocnf_creds['password']
+    elif login_password is None and login_user is not None:
+        module.fail_json(msg='when supplying login arguments, both login_user and login_password must be provided')
+
+    if login_user is not None and login_password is not None:
+        client.admin.authenticate(login_user, login_password)
 
     if state == 'present':
         if password is None: