Bring back (abandoned) support for Cinder/NFS

Bring back (abandoned) support for Cinder/NFS

We used to have a YAML file providing a test setup for Cinder/NFS
which could be used via a special Makefile target; this was not
used in CI anymore though and overtime things broke.

This change aims at bringing that functionality back and also
make it easier to use it via a number of changes:

* delete unmaintained nfs-server-source (not working due to
  changes in the elements)
* delete (unneeded) block-storage-nfs
* remove the hidden block-storage-with-nfs target from Makefile
* add a some nfs-source which supports newer elements and
  newer template language as well
* improve existing comments in Makefile documeting how to use it

Change-Id: I96144ee2f4ca33bd7467f09ad960ea268c1250bf