Adds nfs-server element

This patch adds an element that installs and configures nfs
services for testing the cinder nfs backend in an environment
where no nfs services are present (e.g. CI)

When added to an image, this element creates a directory at
/mnt/state/var/lib/nfs and shares it to each node of the
overcloud.

This patch that handles the package mapping in diskimage-builder
needs to be applied first: https://review.openstack.org/#/c/76398/.

Change-Id: I4c425af64b866caa80b32b808e8a3b97d71ca81d
Merge "Adds nfs-server element"