Clean up container shutdown

1) Don't explicitly clean up emphemeral ZK nodes.  These will go away when
shutdown the ZK connection, and if we lost zk server these function will not
complete.
2) Sanify (as in make sane) healthcheck exit channel handling.
3) Stop performing health checks after container exit has been requested.
Merge pull request #1066 from control-center/feature/shutdownZk

Clean up container shutdown