NFS mounting for Glance file backend

NFS mounting for Glance file backend

Previously we did this with Pacemaker, but with move to NG HA
architecture we lost the ability to use NFS mounts as image storage for
Glance. This reimplements the mounting without utilizing Pacemaker. The
mount is by default also written to /etc/fstab so that it persists over
reboot, but this behavior can be disabled.

This could also go to puppet-glance eventually, but not yet -- we need
this backported to Newton because it's a TripleO regression. I don't
think puppet-glance would allow backporting this to Newton, because from
their point of view it would be a RFE rather than a regression.

Change-Id: I45ad34c36587a8d695069368cf791f1efb68256c
Related-Bug: #1635606