manager: clear stale maintenance node caused by simultaneous cluster shutdown

manager: clear stale maintenance node caused by simultaneous cluster shutdown

Currently, the maintenance node for a service is only cleared when the
service is started on another node. In the edge case of a simultaneous
cluster shutdown however, it might be that the service never was
started anywhere else after the maintenance node was recorded, because
the other nodes were already in the process of being shut down too.

If a user ends up in this edge case, it would be rather surprising
that the service would be automatically migrated back to the
"maintenance node" which actually is not in maintenance mode anymore
after a migration away from it.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>