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"