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: Ifaa37398634f3cda73525363f8e197f03fd63a7d
diff --git a/.zuul.yaml b/.zuul.yaml
index 8430f99..d5b8394 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -121,6 +121,7 @@
           DEFAULT_INSTANCE_TYPE: ds512M
           DEFAULT_INSTANCE_USER: ubuntu
           BUILD_TIMEOUT: 784
+          LIBVIRT_TYPE: kvm
       devstack_services:
         cinder: true