Adding health check endpoint to distribution

Adding health check endpoint to distribution

* Adding health check REST endpoint to distribution service.
* Adding isAlive field to DistributionActivator to fetch the health
  status.
* Adding parameter group for rest server parameters.
* Adding DistributionRestController for hosting all the rest endpoints
  in distribution service.
* Adding DistributionRestServer to manage lifecycle of distribution rest server.
* Adding ParameterValidationUtils utility class for common validations.
  Plan is to move this class to policy-common for wider use later.
* Adding test cases for all new code added.
* Refering common classes from policy/common

Change-Id: I246d57133ed1f0c3548bcdee173d7b64fb368abc
Issue-ID: POLICY-1035
Signed-off-by: ramverma <ram.krishna.verma@ericsson.com>