cephadm: minor reorgs to command_rm_daemon

cephadm: minor reorgs to command_rm_daemon

Make a few small chanes to command_rm_daemon to clean it up in
preparation for future changes. Move the construction of a
DaemonIdentity earlier and use the service name derived directly from it
if possible. I didn't remove the old approach of looking up a service
name in the listing because I don't know if doing so would break
old/adopted services and there's no unit test coverage for that.

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