Support NFS backend for Glance (via Pacemaker)

Support NFS backend for Glance (via Pacemaker)

Adds support for NFS backend in Glance by allowing the storage directory
for the 'file' backend to be a mount managed by Pacemaker. Default
behavior is unchanged.

Since the Pacemaker-related parameters are not exposed on top level,
change storage-environment.yaml to use parameter_defaults instead of
parameters.

Depends on a Heat fix for environment file's parameter_defaults to
work well with JSONs and comma delimited lists (see Depends-On).

Change-Id: I6e7e2eaf6919b955650c0b32e1629a4067602c89
Depends-On: I85b13a79dbc97a77e20c0d5df8eaf05b3000815e