Introduce support for BLOCKSTORAGESCALE

This change introduces support for BLOCKSTORAGESCALE which initially
allows for the deployment of additional cinder-volume nodes, all
using the LVM driver, in addition to the controller nodes.

Summary of changes:
  - documentation is updated to mention existence of BLOCKSTORAGESCALE
  - additional overcloud-cinder-volume image is (conditionally) built

Change-Id: Id7bc4c2e66b97f109f1ea91e5e3e45d02eddb3cc
Merge "Introduce support for BLOCKSTORAGESCALE"