Add identity_feature_enabled.xml_api config

This change allows us to continue to test identity XML support for
stable icehouse. XML support in Keystone has been deprecated and staged
for removal in Kilo.

Change-Id: Ibb402640c5b744f1ea38ce548994079996c72508
Merge "Add identity_feature_enabled.xml_api config" into stable/icehouse
Merge branch 'stable/icehouse' of https://github.com/openstack-dev/devstack into stable/icehouse