Support --os-version as argument.

Support --os-version as argument.

You can use --os-type as an argument when not running teuthology
tests but instead just using teuthology-lock. This adds the ability
to also use --os-version so you can specify the version of the
distro without having to run an actual test with a yaml like you
normally would have had to do setting os_version in the yaml.

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