mgr/cephadm: verify mon spec exists before trying to grab from spec store

mgr/cephadm: verify mon spec exists before trying to grab from spec store

In a normal deployment, we generally shouldn't have
to worry about this. This is more for teuthology
which does deployments in a weird way that can cause
there to be no mon spec in the cluster. Fixes an issue
seen when backporting the mon crush location work
to quincy where an upgrade test would fail with

```
[WRN] UPGRADE_REDEPLOY_DAEMON: Upgrading daemon mon.b on host smithi047 failed.
    Upgrade daemon: mon.b: Service mon not found.
```

Signed-off-by: Adam King <adking@redhat.com>
(cherry picked from commit 8aab7beefbb52f47573ad7fce932552ad5c0b2fa)