In order to support switching auth contexts, such as for registered_limits
which take a system scoped token, switch the functional tests to using
the --os-cloud command line parameter. However, honor the OS_CLOUD env var
as a way that someone can select a different cloud, including 'envvars',
to use.
Use devstack-system-admin cloud for limit tests
Keystone requires these to have system scope now.
Change-Id: Ia81eebd3e00ae986cf3ba7e3d98f3e8a1647b622
2dd5393167