Fix testTransactionForwardedToLeaderAfterRetry failure

Fix testTransactionForwardedToLeaderAfterRetry failure

java.util.concurrent.ExecutionException: ReadFailedException{message=Error executeRead ReadData for path /(urn:opendaylight:params:xml:ns:yang:controller:md:sal:dom:store:test:cars?revision=2014-03-13)cars/car, errorList=[RpcError [message=Error executeRead ReadData for path /(urn:opendaylight:params:xml:ns:yang:controller:md:sal:dom:store:test:cars?revision=2014-03-13)cars/car, severity=ERROR, errorType=APPLICATION, tag=operation-failed, applicationTag=null, info=null, cause=org.opendaylight.controller.md.sal.common.api.data.DataStoreUnavailableException: Shard member-1-shard-cars-testTransactionForwardedToLeaderAfterRetry currently has no leader. Try again later.]]}

The test submits transactions and deposes the current leader so it forwards the
pending transactions to the other member-2 that assumes leadership. However it calls

   Cluster.get(followerSystem).leave(MEMBER_1_ADDRESS);

which may result in an untimely MemberExited message sent to the ShardManager that
clears the peer address, causing the FindPrimary message to fail to find the leader.
I'm not clear why this was call was put in but it's unnecessary and may cause a
failure if the timing is right.

I also saw a failure due to a timeout when forwarding a pending transaction. This is
b/c it takes some time for member-2 to switch to candidate and become leader due to
the checking of current leader availability via the akka cluster on ElectionTimout.
If it takes too long the pending transaction forwarding may time out. To alleviate
this, I forced the swicth to candidate by sending an immediate TimeoutNow message.

Change-Id: I2dd228964779e2b755b1740a518e2c400b5cb88d
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>