Fix external_network_bridge interpolation error

The external_network_bridge fails to interpolate unless
the value is set in global_hiera_params. Setting it
back to br-ex until we have a more elegant way to
override this from common.

Change-Id: Ia045b05b8e67e5422ac4007f4c5385e636dbe41e
Merge branch 'master' of https://github.com/stackforge/puppet_openstack_builder into icehouse