Move baremetal API tests to an /admin subdir

To avoid confusion, move all baremetal API tests to api/baremetal/admin and
add a README.rst explaining that the API provided by Ironic is meant to be used
as an admin and service level API.

Change-Id: Ib49b6d5937da6eedc3028a1f831552cf02d9f69e
Merge "Move baremetal API tests to an /admin subdir"