reset failed count of ceph-mgr

reset failed count of ceph-mgr

Depending on your setup, ceph-mgr might get restarted multiple times.
When this is done to fast, systemd will prevent further restarts because of
configured limits in the ceph-mgr systemd unit file.

Resetting the failure count will prevent this problem. The reset is done before
the restart so in case of a real problem during the restart it still fails.

Fixes: #2768
Signed-off-by: Christian Zunker <christian.zunker@codecentric.cloud>