Fix intermittent test failures in ClusterAdminRpcServiceTest

Fix intermittent test failures in ClusterAdminRpcServiceTest

Failed tests:
  ClusterAdminRpcServiceTest.testChangeMemberVotingStatesForShard:555->verifySuccessfulRpcResult:296
Rpc failed with error: RpcError [message=Failed to change member voting
states for shard cars: Shard
member-3-shard-cars-config_testChangeMemberVotingStatusForShard
currently has no leader. Try again later., severity=ERROR,
errorType=RPC, tag=operation-failed, applicationTag=null, info=null,
cause=null]

Needs to ensure node3's datastore shards are ready with leaders.

Change-Id: Iae6179e6f577b98f267c1afd3a901a14eed81e7f
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>