Added missing important step to Glance config

The section covering the use of vCenter datastores for the Glance
backend was missing a key step about creating a storage profile.
This section has been updated to include this step. I also added
the stop chunk command to the vmdk driver doc to keep the sections
together.

Change-Id: I68d55ba6948d1083032a8ee347b81d2b4f63e980
backport: icehouse
Closes-Bug: #1320369
(cherry picked from commit f3926323aba6bb19958bf4c89d24e982cb4930fa)