Rewrite the release instructions to use a simpler workflow.

[porridge]

This depends on https://github.com/mesosphere/data-services-kudo/pull/64 and should be rebased after it's merged.

This changes the release workflow to use another approach, where we simply create the branch and tag via GitHub UI rather the (IMHO) overly complicated Python script that strangely uses Golang conventions for error handling.

• Add a couple of previously unmentioned steps, including one to build and push docker images using a TeamCity job.

• Tolerate unset $IMAGE_DISAMBIGUATION_SUFFIX.

• Use the disable-disambiguation-suffix branch of shared submodule.

• Remove unused release script.

[porridge]

Kind ping for a review.

[porridge]

FTR this is blocked on https://github.com/mesosphere/kudo-cassandra-operator/pull/129