devtest_overcloud: boot an instance from a volume

This patch updates our nova boot command to boot
an instance from a volume that is created on
the fly from a Glance image.

The motivation here is to expand our devtest
script so that it has some level of coverage
across Cinder which is commonly broken
in upstream TripleO.

Change-Id: Icd50c78f9d3cc11064e92dbd27a55235b86ceb25
Merge "devtest_overcloud: boot an instance from a volume"