Mongo DB connector should properly validate if password and rights

During UI testing we found that a Mongo DB connector config is reported as valid, but the connector won't be able to start afterwards. This might be caused by insufficient credentials and user rights checks in MongoDbConnector#validate.

• add positive and negative (should fail when invalid credentials are supplied -> should fail with current implementation) tests for the above scenario
• fix MongoDbConnector#validate checks if the provided replica set/s and schemas can be accessed with the provided config