Prevent virbr0 from being created by default

Small change to remove the default network since neutron handles all
of the bridge and veth creation. virbr0 becomes clutter.

CentOS doesn't seem to come with this default.xml network, so no
change there.

Note: This doesn't remove virbr0 from a running system, it only
prevents its creation.

Change-Id: I8e118ea285c674a47884bbe864df134133406bbc
Closes-Bug: #1512060
Potential-Backport: Liberty

docker/nova/nova-libvirt/Dockerfile.j2

@@ -24,7 +24,8 @@ RUN apt-get install -y --no-install-recommends \
         qemu-block-extra \
         ebtables \
     && apt-get clean \
-    && mkdir -p /etc/ceph
+    && mkdir -p /etc/ceph \
+    && rm /etc/libvirt/qemu/networks/default.xml /etc/libvirt/qemu/networks/autostart/default.xml
 
 {% endif %}