Set up functional gate tests for searchlight

It's particularly crucial that RBAC works as expected in an actual deployment (as opposed to just unit tests). We need a decent set of functional tests that test this and other functionality to prevent regression and ensure searchlight functions properly with changes to dependent projects.

There exists a review in glance with a starting point (https://review.openstack.org/#/c/157209/) but it will likely need some changes. In particular, it's non-trivial to run tests in parallel since elasticsearch isn't easily replaceable by an sqllite equivalent; for this reason it may be useful to make plugin index names customizable (which should probably be done in another blueprint).

This will require some interaction with the infra team, and probably also devstack integration. Tests should be non-voting until they're verified to be working properly at which point they should be voting.