This resolves one of the TODOs in the heal_allocations CLI
by adding an --instance option to the command which, when
specified, will process just the single instance given.
NOTE(mriedem): The "since version" part of the CLI docs change
is removed here since it does not make sense to mention this
is a new option since Train in the Stein docs.
Change-Id: Icf57f217f03ac52b1443addc34aa5128661a8554
(cherry picked from commit c92b297896)
92645be4bb