Removed calls to ensurePath

Zookeeper operations must fail if the ZK cluster ID
is missing or wrong. This makes sure agents only talk
to ZK clusters they are intended to. With this model
we cannot call ensurePath, because that would create
the expected path. Instead we must create individual
nodes under an existing path.
Bootstrapping script.

Added a script to bootstrap a new Helios cluster using namespaces and stuff.
Drop namespace support in Bootstrap.
Merge branch 'kobolog/no-tombstone-agent' of https://github.com/spotify/helios into kobolog/no-tombstone-agent
Revert ClusterId check in PathChildrenCache.