set xml_api flag for Network tests

XML api support is being removed in Kilo, so we need to enable here to
ensure that the API is still tested in this stable release.

Change-Id: Iaf0cb93fda607c62911c57132eced14a33f94f97
Merge branch 'stable/icehouse' of https://github.com/openstack-dev/devstack into stable/icehouse