Externalize hooks for neutron functional jobs

Maintaining hook behavior in the config repo has the drawback of
being hard to test.  This change externalizes the hook definitions
for the neutron functional jobs so they can be maintained and
gated on via the neutron repo.

The supporting neutron change to master:

https://review.openstack.org/#/c/114717/

Change-Id: If5192cb22b5613c5ebcab63316c646ea3ab482df
Merge "Externalize hooks for neutron functional jobs"