mgr/nfs: limit dependency of NFSRados object

mgr/nfs: limit dependency of NFSRados object

Previously, the NFSRados object accepted the "Module" as the
first argument but only used the rados attribute (type rados.Rados).
It's better to limit the scope of types when reasonably possible
so we can see what the true dependencies are. So we restrict
NFSRados to accepting a rados.Rados as the argument.

Signed-off-by: John Mulligan <jmulligan@redhat.com>