In Pike, Cinder and Nova will support extending the size of
an attached volume, but it's not supported by all volume and
compute backends. Tempest will also test it but uses a config
option that we need to set based on how devstack is configured.
Depends-On: Ibace6c2f91be9753a44e5f79fd013df11654851b
Related to cinder blueprint extend-attached-volume
Related to nova blueprint nova-support-attached-volume-extend
Change-Id: I52cc2952a2938ce44c442aa3e3b69a905b2b55d5