Enable libvirt virtio rng device

We've seen jobs where tests fail due to what appears to be rng
starvation. Enable virtio rng device to try and alleviate this.

Change-Id: I70d800cdc45b6008f775110f22c0000736421529
diff --git a/lib/tempest b/lib/tempest
index 96c9ced..1bb9b52 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -203,13 +203,13 @@
             if [[ ! ( $available_flavors =~ 'm1.nano' ) ]]; then
                 # Determine the flavor disk size based on the image size.
                 disk=$(image_size_in_gib $image_uuid)
-                openstack flavor create --id 42 --ram 64 --disk $disk --vcpus 1 m1.nano
+                openstack flavor create --id 42 --ram 64 --disk $disk --vcpus 1 --property hw_rng:allowed=True m1.nano
             fi
             flavor_ref=42
             if [[ ! ( $available_flavors =~ 'm1.micro' ) ]]; then
                 # Determine the alt flavor disk size based on the alt image size.
                 disk=$(image_size_in_gib $image_uuid_alt)
-                openstack flavor create --id 84 --ram 128 --disk $disk --vcpus 1 m1.micro
+                openstack flavor create --id 84 --ram 128 --disk $disk --vcpus 1 --property hw_rng:allowed=True m1.micro
             fi
             flavor_ref_alt=84
         else