Add config option for XML support in Networking

The first step to removing XML in Kilo is making XML testing optional.
Stable test configs will need to be updated to enable this feature to prevent
regressions in stable releases.

Change-Id: I30b9565e01c623814a9c503c4b1c26642afed878
Merge "Add config option for XML support in Networking"
Merge branch 'master' of https://github.com/openstack/tempest