Enable parallel building
Enable parallel building

This theme provides a sphinx extension that is used with parallel
building, thus enable parallel building so that we can sure it works
everywhere.

The time benefit is not worth the work, this is for testing.

Change-Id: I4e8b7fdb5c4b8c6bd738211802567b532528ab9a

openstack/openstackdocstheme,openstack/openstackdocstheme,openstack/openstackdocstheme,openstack/openstackdocstheme