mgr/nfs: support managing exports without orchestration enabled

mgr/nfs: support managing exports without orchestration enabled

This change allows the `ceph nfs export ...` commands to function
without the entire mgr/nfs subsystem requiring orchestration to be
enabled.  When there's no orchestration available, the code falls back
to examining the namespaces in the ".nfs" rados pool to determine what
cluster_id values are valid.

This change does not add support for creating the rados objects and
namespace needed to manage a nfs cluster. As discussed with the
orchestration group on 2022-01-22, rook does not need the mgr module to
establish the namespace. So, for now, we'll defer the work needed to
create the namespace/objects when orchestration is disabled.

Fixes: https://tracker.ceph.com/issues/54043
Signed-off-by: John Mulligan <jmulligan@redhat.com>