FR: allow disabling individual releaseable units in a group/bulk release config

[shollyman]

Hi. Quick feature request about controls for scenarios where a single release config controls multiple releaseable units.

It would be nice to embargo a particularly releasable unit from inclusion in a bulk release.

A simple straw man:

• In google-cloud-go, we have a bulk release with N independent modules.
• Sometimes, a given module is getting manual PRs that modify the behavior and those changes may be over an extended period (e.g. longer than the typical 1/week release cycle).
• We want to release everything BUT the affected module, rather than delaying all the modules from being released.