Use nested virt in scenario jobs

virt_type option in nova-compute will be now set to "kvm" instead
of "qemu" if test job will be running on node which supports
nested virtualization.
In case of nodes where it's not supported, devstack will
automatically switch it to "qemu" again. It's in [1].

This should improve time of booting vms so tests should be finished
faster and there should be less errors with ssh to instance timeouts.

[1]
http://git.openstack.org/cgit/openstack-dev/devstack/tree/lib/nova#n289

Change-Id: Ia361f33f0b103ed44ef372c1464b3c0473a19291

neutron/tests/contrib/gate_hook.sh

@@ -106,6 +106,9 @@ case $VENV in
     load_conf_hook osprofiler
     load_conf_hook availability_zone
     if [[ "$VENV" =~ "dsvm-scenario" ]]; then
+        # Use nested_virt, if node don't support it, Devstack's will
+        # automatically switch it back to "qemu"
+        export DEVSTACK_GATE_LIBVIRT_TYPE=kvm
         load_rc_hook ubuntu_image
     fi
     if [[ "$VENV" =~ "dsvm-scenario-linuxbridge" ]]; then