BUG 2185 : Make the Custom Raft Policy externally configurable

BUG 2185 : Make the Custom Raft Policy externally configurable

A class which implements RaftPolicy needs to be set
to customize the behavior. I am hoping that this will be a barrier
to people unintentionally breaking the default raft policy as
they would need to know the internals of clustering in order to
supply a valid class.

I added two configurable raft policy classes for the two known
use cases,

- org.opendaylight.controller.cluster.datastore.policy.TestOnlyRaftPolicy
- org.opendaylight.controller.cluster.datastore.policy.TwoNodeClusterRaftPolicy.

Change-Id: Ic3cc2f27754c37e85c3be8a863764fc88ec84399
Signed-off-by: Moiz Raja <moraja@cisco.com>
(cherry picked from commit 1ef2f7321202ef77b5cfd0ca430e6a58cb07331e)