mgr/cephadm: Fix race between host_add and _apply_all_specs

mgr/cephadm: Fix race between host_add and _apply_all_specs

This mitigates a potential race, where new host was added *after*
``_refresh_host_daemons()`` was called, but *before*
``_apply_all_specs()`` was called. thus we end up with a hosts
where daemons might be running, but we have not yet detected them.

Imagine:

1. the initial MGR was deployed manually
2. `serve()` is called
3. `_refresh_hosts_and_daemons()` is called
4. `add_host()` is called with the initial host
5. `apply_all_specs()` now deploys an additional MGR on the initial host

Fixes: https://tracker.ceph.com/issues/45093
Signed-off-by: Sebastian Wagner <sebastian.wagner@suse.com>