mgr/cephadm: fix NFS ganesha service registration via nodeid and register_service

mgr/cephadm: fix NFS ganesha service registration via nodeid and register_service

The NFS Ganesha service was not consistently visible in `ceph -s`,
especially in multi-daemon deployments. This was due to missing or
incorrect service registration with the Ceph manager.

This change updates the ganesha.conf template to explicitly set:

  CEPH {
      register_service = true;
      nodeid = "{{ namespace }}.{{ nodeid }}";
  }

Key points:
- Enables proper service registration in mgr via register_service
- Ensures unique nodeid per daemon using namespace + nodeid
- Fixes visibility of NFS daemons in `ceph -s`
- Works correctly for both single and multi-node deployments

Validation:
- Verified with single NFS daemon → visible in `ceph -s`
- Verified with 3 NFS daemons → all correctly aggregated and visible
- Confirmed export creation activates service visibility
- Tested using Ceph 9.1 (Ganesha 9.7)

No regressions observed.

Fixes: https://tracker.ceph.com/issues/75709
Signed-off-by: Shubha Jain <SHUBHA.JAIN1@ibm.com>