Minimize service installation for neutron func job

The neutron functional job only needs devstack to install
its dependencies, and there is no need to install other
services.

This change depends on the following openstack-infra/devstack-gate
changes:

https://review.openstack.org/#/c/116892/
https://review.openstack.org/#/c/117279/

Change-Id: I54a10daa9bd54936b1ee09f344e60cf88ea76f4c
Merge "Minimize service installation for neutron func job"