Send Shutdown message to Shard on ServerRemoved

Send Shutdown message to Shard on ServerRemoved

Modified the ShardManager to send the Shutdown message to the Shard on
ServerRemoved. If the shard was the leader, this will trigger leadership
transfer.

I also made changes to propagate the appropriate error to the caller on
RemoveServerReply instead of always replying with success.

Added a test case in ClusterAdminRpcServiceTest for removing the leader.

Change-Id: I30d2a22f07c1003fad2aba68e4f2d1d2c9fe7eb3
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>