qa: fix cluster creation failure in test_nfs.py

qa: fix cluster creation failure in test_nfs.py

Also adds a function _nfs_complete_cmd() that returns process obj so that stdout/stderr
can be used for evaluation(_nfs_cmd() uses raw_cluster_cmd() that returns just stdout
and it became difficult to time cluster creation errors in _test_create_cluster()).

It takes sometime to update the cluster data, therefore running the command set
(check nfs server status -> nfs cluster create test -> check cluster status) in
a loop (max six iteration with sleep of 5 secs at each iteration) fixes the issue.

Fixes: https://tracker.ceph.com/issues/58744
Signed-off-by: Dhairya Parmar <dparmar@redhat.com>