this patch moves logic for preparing libvirt environment
and creating test VMs out of bash script to Ansible tasks
using virt_* modules available since Ansible 2.0.
The only virsh command left is volume creation as Ansible still
lacks modules for libvirt volume management.
As a consequence, many variables hidden in the bash script are now
explicitly defined in defaults of 'bifrost-create-vm-nodes' role and can
be redefined by standard Ansible means.
Although the libvirt connection string can be redefined too,
it was not tested with any other than default value (local qemu connection).
Change-Id: Idaca6b5f502e0e2391af92651f30ab9dcd2fbb3b
Related-Bug: #1659876
f888748d4e