mon/MgrMonitor: blacklist previous instance

mon/MgrMonitor: blacklist previous instance

This wasn't realized as necessary early on in the ceph-mgr development
because the mgr didn't interact with RADOS. However, now it is becoming
common for plugins to store data there. It's important that the previous
instance can no longer interact with RADOS while the new mgr takes over.

In particular, this means that the mgr's client sessions with the MDS
are automatically evicted once the MDS receives the new OSDMap. This
avoids a pesky "unresponsive client" warning in the cluster logs.

Fixes: https://tracker.ceph.com/issues/42939
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>