Remove obsolete zk Docker example.

The useful parts of the zk Docker example have been absorbed into the
Kubernetes example. The Docker-only version is now broken anyway and
there's no point in fixing it since we're not planning to flesh out the
rest of Vitess in Docker-only mode.
Allow tablet hostname to be overridden.

Kubernetes sets a hostname for each pod that can't be changed
dynamically, and can't be resolved by other pods. We need to tell
vttablet to report its hostname in the topology as something
globally resolvable (or as a fixed IP), instead of asking the OS.
Merge pull request #125 from youtube/docker

Allow tablet hostname to be overridden.