Do not run multiple tests (for distros) on baremetal.

Teuthology doesnt care about os_type for baremetal (ATM). This
change makes it so you can run tests that have been switched over
to run on multiple distros (on vms) on baremetal as well as all
non-ubuntu tests will be skiped (to avoid running the same test
multiple times on baremetal.

Signed-off-by: Sandon Van Ness <sandon@inktank.com>