Handle RC milestone composes correctly
The CLI and `get_release` did not handle composes for the 'RC'
milestone correctly. With Pungi 4, the milestone for GA composes
is now 'RC', not 'Final'; we were handling this OK when parsing
a label or compose ID, I think, but the CLI and `get_release`
in old-style 'release, milestone, compose' usage were both
expecting 'Final' as the milestone (that's what we called these
before Pungi 4). So have `get_release` check for 'Final' and
convert it to 'RC', and have the CLI accept both 'Final' and
'RC' as synonyms. Tweak a test to cover this, and improve the
CLI help text while we're at it.