Add maintenance mode to the Mistral

Add maintenance mode to Mistral
Using maintenance mode user can manage state for the mistral deployment which performing maintenance activities
1) User can pause mistral deployment by setting maintenance mode to paused for any maintenance activities.
2) Once mistral cluster is paused, all the running executions will be paused and only GET requests will be allowed to read mistral data.
3) Add configurable option to allow specific PUT/POST API calls when Mistral deployment is paused.
4) When Mistral deployment is resumed after performing maintenance activities, all the paused executions will be resumed.