mgr/cephadm: migrate all legacy nfs exports to new .nfs pool

mgr/cephadm: migrate all legacy nfs exports to new .nfs pool

Migrate all past NFS pools, whether they were created by mgr/nfs or by
the dashboard, to the new mgr/nfs .nfs pool.

Since this migrations relies on RADOS being available, we have to be a bit
careful here: we only attempt the migration from serve(), not during
module init.

After the exports are re-imported, we destroy existing ganesha daemons so
that new ones will get recreated.  This ensures the (new) daemons have
cephx keys to access the new pool.

Note that no attempt is made to clean up the old NFS pools.  This is out
of paranoia: if something goes wrong, the old NFS configuration data will
still be there.

Signed-off-by: Sage Weil <sage@newdream.net>