Set public bridge up for v6 only configurations

A long time ago, Ironic's IPv6 only job started to fail working with
errors indicated the host was unreacable. Turns out, this was because
the $ext_gw_interface was not being set to up, and thus could
be found in a Down state, and thus the kernel would not accept routes
for it.

Adds an explicit step to turn up the public bridge, much as done in
the IPv4 router plugin code which would also be executed in 4+6.

That being said, Ironic's CI jobs are very intentionally IPv6 only
to ensure that we have no chances of v4 addressing getting used
at any point in time.

This should allow Ironic to return it's IPv6 only CI job back
to the normal check queue, once a ironic plugin issue has been
resolved which was introduced while it was removed.

Change-Id: I121ec8a2e9640b21a7126f2eeb23da36b4aa95bf

lib/neutron_plugins/services/l3

@@ -403,7 +403,10 @@ function _neutron_configure_router_v6 {
             ext_gw_interface=$(_neutron_get_ext_gw_interface)
             local ipv6_cidr_len=${IPV6_PUBLIC_RANGE#*/}
 
-            # Configure interface for public bridge
+            # Configure interface for public bridge by setting the interface
+            # to "up" in case the job is running entirely private network based
+            # testing.
+            sudo ip link set $ext_gw_interface up
             sudo ip -6 addr replace $ipv6_ext_gw_ip/$ipv6_cidr_len dev $ext_gw_interface
             # Any IPv6 private subnet that uses the default IPV6 subnet pool
             # and that is plugged into the default router (Q_ROUTER_NAME) will