BUG 2134 : Make persistence configurable at the datastore level

- Added a peristent flag in the config sub-system
- If persistent in not true then we ignore recovery messages and not use the
  akka persistence apis for persisting anything
- The unit tests
    - assume that persistence is on by default (which it is)
    - test that if recovery is applicable only then the ShardManager and RaftActor process the recovoery messages.
    - test that when persisting the data-persistence API is used in the appropriate places (see RaftActorTest/ShardManagerTest)

Change-Id: I19913bcd32e609ccde6ad8e35209788315504426
Signed-off-by: Moiz Raja <moraja@cisco.com>