handle zookeeper action when some node is gone
send nominate_master and change_master when master node is dead
Merge branch 'master' of github.com:DiMint/DiMint_Overlord
enable max slave count in config file
Merge branch 'master' of https://github.com/DiMint/DiMint_Server into issue#20

Conflicts:
	dimint_overlord/Overlord.py
rebalance: determined which keys to move