The tempest plugin expects the classic environment variables
to be present for credentials to access the cloud, but this is
wrong in cases where we're trying to setup system
scoped services and need to remove the environment variables
that was being used.
Instead, change the plugin to use the os-cloud entry definitions,
and specifically in this case devstack-admin which makes sense
until we begin to start to make tempest itself scope aware.
We likely will want to change the environment variables from being
registered in devstack at some point and completely shift towards
passing an-os-cloud parameter, but that is outside the scope of
this change as doing so will likely break all plugins.
Change-Id: I8d4ec68f116eea07bc7346f939e134fa2e655eac
5c9affdd9a