Update toggle variable for HTTPD + mod_wsgi services

Update the devstack-gate toggle variable to match the new global
toggle for running services under alternatives to HTTPD + mod_wsgi.
This toggle only affects services that default to running under
HTTPD + mod_wsgi.

The new toggle was added in: https://review.openstack.org/#/c/101611/

Change-Id: I7ebaebf7806049fee2a6e57df628ad934b6d494c
Merge "Update toggle variable for HTTPD + mod_wsgi services"