Add a section on NFS storage backend setup

This patch adds instructions on how to configure an NFS Storage
Backend to the Cloud Admin Guide.

Change-Id: I7a21a3cad23aada47ab48ff149c619b99e08de29
Closes-Bug: #1280951
Added instructions for GlusterFS share removal

Removing a GlusterFS share is not as simple as deleting its entry from
the shares file. This patch adds instructions on how to gracefully
and safely do so.

Change-Id: I50fd39dfcb147de42c9295ceae64e55b56aeb265
Partial-Bug: #1279179
Merge "Added instructions for GlusterFS share removal"
Merge "Add a section on NFS storage backend setup"