Plugins can't be disabled in build

If a user is building an image, neutron-server for example, and doesn't wish to use tarballs.openstack.org for the source as provided by kolla/common/config.py, they can simply override this in kolla-build.conf. However, if they don't wish to pull in all plugins provided for neutron-server by config.py, there is no way to do this.

One solution is to add an 'enabled' argument to these that can then be toggled in kolla-build.conf.