Add additional instructions for cinder install

the volume creation was not documented for cinder install

this patch adds it.

Backport: stable/havana

Cherry-Picked from: https://review.openstack.org/#/c/52572/

Change-Id: I70e2c2a537ce35225fbf4210b05be06052431246
Merge "Add additional instructions for cinder install" into stable/havana