mgr/cephadm: add tombstones to persist certs info after mgr failover

mgr/cephadm: add tombstones to persist certs info after mgr failover

Runtime-added TLS objects names were lost across mgr restarts/failovers
since they existed only in memory. We now write a tombstone to the KV
store whenever a new certificate is registered (empty map for
service/host scope; minimal JSON for global), so the object name is
restored during load().

Fixes: https://tracker.ceph.com/issues/73625
Signed-off-by: Redouane Kachach <rkachach@ibm.com>