Fix usage of CinderNfsServers

Fix usage of CinderNfsServers

This feature stopped working somewhere along the lines. In the past it
was working with parameter_defaults like this:

 CinderNfsServers: '10.0.0.254:/srv/nfs/cinder'

or

 CinderNfsServers: "[fd00:fd00:fd00:3000::1]:/srv/nfs/cinder"

The problem was that the templating escaped these strings, and
puppet-tripleo didn't receive a proper array, but a string.

This patch fixes this. It accepts strings as above as well as
comma-delimited lists of Nfs Servers.

Closes-Bug: 1671153
Change-Id: I89439c1d969e92cb8e0503de561e22409deafdfc