mgr/dashboard: directly use ExportMgr and NFSCluster objects

mgr/dashboard: directly use ExportMgr and NFSCluster objects

Using the objects directly provides access to other methods and helps in
avoiding repeatition.

mgr/dashboard/nfsganesha: remove tag

Since NFS v3 is no longer supported. We can remove tag.

mgr/nfs: define global constant to list supported FSALs

mgr/dashboard: directly list nfs clusters by directly importing available_cluster() method

The current dashboard api returns a list of following dictionary

{
   'pool': 'nfs-ganesha',
   'namespace': cluster_id,
   'type': 'orchestrator',
   'daemon_conf': None
}

None of these values are required for listing nfs cluster by mgr/nfs module.
Instead directly list available cluster names

mgr/dashboard: add comment to remove listing of daemons

As the configs are per cluster. There is no need to list daemons per cluster.

mgr/dashboard/controllers/nfsganesha: Add comments to update/remove status endpoint

This endpoint can be updated in suggested way or even removed. As it was
initially[1] introduced to check if dashboard pool and namespace configuration was
set.

[1] https://github.com/ceph/ceph/commit/824726393b185b8e5a8f17e66487dfde9f3c8b5c

mgr/nfs: remove fetch_cluster_obj()

There is no need to fetch NFSCluster class object. Directly
available_clusters() can be imported to list nfs clusters.

mgr/dashboard/controllers/nfsganesha: list exports based on cluster id

As mgr/nfs module lists based on cluster id.

mgr/dashboard/nfs: get and delete export by export id

Fixes: https://tracker.ceph.com/issues/46493
Signed-off-by: Varsha Rao <varao@redhat.com>
(cherry picked from commit 64178e1f5b0f26f5c17898e85c3d5b439e5f9dbf)