Ensure testing is against current commit
Ensure testing is against current commit

By using the centralized testing repo and its ansible.cfg file, a git
clone of the plugins repo is made to ~/.ansible/ and used for testing.
Effectively, testing of plugins is only done against the last merged
commit.

Set environment variables through tox so that ansible uses the library
and plugin paths in tree.

Change-Id: I61d60ae997d1bbcfdfa154a68908f7131ebc11c3

os-cloud/openstack-ansible-plugins,os-cloud/openstack-ansible-plugins,openstack/openstack-ansible-plugins,openstack/openstack-ansible-plugins