Use cirros image for overcloud testing

There is no need to build a fully operational image for testing
the overcloud in TripleO. A simple cirros image, with a smaller
memory and disk footprint and with no need to be built using dib
will accelerate the testing.

Change-Id: Iafac7ca4f18fd4734ae242242f73395ecbae80d6
Merge "Use cirros image for overcloud testing"