add health checks to operator manager

Fixes #212 .

added health checks for liveliness and readiness in operator manager deployment.

This PR has:

[x] been tested on a real K8S cluster to ensure creation of a brand new Druid cluster works.
[x] been tested for backward compatibility on a real K*S cluster by applying the changes introduced here on an existing Druid cluster. If there are any backward incompatible changes then they have been noted in the PR description.
[ ] added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
[ ] added documentation for new or modified features or behaviors.

druid-io / druid-operator