Provide documentation on etcd-operator-builder. Remove references to gcr.io image.

[pjsully]

This addresses #2156 and likely will close it.

if there is a need to use the builder image just build it locally and push it where you need it.

Added documentation for generating builder and using it.

[pjsully]

Note that for the ci build script to work, etcd-operator-builder:latest has to be available from the local docker registry...not ideal, but gives a way to make this work (and steps to build the builder are part of this PR as well...)