Provide rollback information for EOC and IMs

During a recent incident, it would have been useful to have been able to rollback several deployment packages to one from before the problem was introduced.

Let's use this issue to work out what would be the best way to provide rollback feasibility information to the SRE on call and the Incident Manager so they can review their options for resolving the incident.

Implemented

The /chatops run rollback check command was extended to allow an option called --target. If an auto deploy version is passed with the --target option, the command checks if the environment can be rolled back to the given version.