Add spec to support running functional tests as different users

This spec details a proposal for the ability to run one (or more)
Barbican functional tests under a userid that is different
from the default.

Change-Id: I1c5e18faf73d26c2b34b9bf59e88ab86ab24a4a3
Implements: blueprint https://blueprints.launchpad.net/barbican/+spec/add-run-as-for-functional-tests
Merge "Add spec to support running functional tests as different users"