clean up zookeeper path for a runnable instance in the application
master before launching the container instead of during container
start up.  This is because an instance can get a message while it
is still starting up.  Cleaning the path during start up would then
delete the message before getting a chance to process it.
Merge pull request #72 from continuuity/feature/clean-zk-before-launching-container

clean up zookeeper path for a runnable instance in the application