mgr/cephadm: add a new Action enum class

mgr/cephadm: add a new Action enum class

Currently, the code base mostly uses strings to represent actions
(like 'start', 'redeploy, 'reconfig', ...). Add a new Action class
to utils.py to have a crisper typing-friendly representation for
actions.

This is added to support net new code. While I'd like to see this
used more the goal is to prevent errors in the new approach and I
don't plan on immediately going back through all of cephadm
and finding and replacing all action strings right away.

Signed-off-by: John Mulligan <jmulligan@redhat.com>