Make ceph-nfs configurable

Ceph-nfs image is not available for Debian build. Because there is
no Debian package under official ceph-nfs' repo:
http://download.ceph.com/nfs-ganesha/deb-V2.5-stable/luminous/dists/

Change-Id: I35d84296318a849616e72fb6a2064b8ab62fdaf7