mgr/cephadm: persist cached daemon state

mgr/cephadm: persist cached daemon state

- load cached state on startup
- persist state after a scrape only
- scrape everything after startup

Note that we modify our in-memory cache when we add or remove a service
and then immediately trigger a scrape, but we do not invalidate the
persisted state, since it's simpler (and presumably a good idea) to
simply re-scrape everything after a mgr restart.

Signed-off-by: Sage Weil <sage@redhat.com>