Apply review edits to GlusterFS+NFS backend setup

This patch applies several editorial fixes to the GlusterFS and NFS
backend setup sections. These fixes were part of the editorial review
process in https://review.openstack.org/#/c/74553/ and
https://review.openstack.org/#/c/73931/ (both of which are merged).

Change-Id: I73f823190913acb755b3362e73db70d09e5b3426
Related-Bug: #1281444
Related-Bug: #1280951
Merge "Apply review edits to GlusterFS+NFS backend setup"