Consolidate neutron-tempest-plugin-api with scenario jobs

That way we will run our API tests on the ML2/{OVS,OVN,Linuxbridge}
deployments which will give us slightly better coverage in some cases as
some API extensions may be available only with some specific backends
(see NDPProxy for example, which isn't available on the ML2/OVN case).

This will also allow us to drop neutron-tempest-plugin-api job and
having less jobs with same or better coverage is always good thing :)

Change-Id: Ie34e89636c56783000b4d72a479c23d8d1295002
13 files changed