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>
Fixed errors. Tests pass.

Since the default OS version is different for each distro the
argument default is None instead of explicity set to a value
like with get_distro. Fixed some logic around that and the tests
making the arugment always take precidence.

Signed-off-by: Sandon Van Ness <sandon@inktank.com>
Consolidate two excepts into one.

Signed-off-by: Sandon Van Ness <sandon@inktank.com>
Merge pull request #146 from ceph/wip-os-type

Wip os type