Add pip_install_options to test vars to ensure libvirt-python is built
from source. A pre-built wheel can be missing libvirt capabilities
from the installed version of libvirt-bin, leading to nova-compute
failing to start.
neutron_rabbitmq_* test variables have also been included as they are
required for the neutron install to complete.
Change-Id: I57e9c50bf835db1ffda2507764a09cb74d7c0884