Allow OS version over-ride (distro version)
Allow OS version over-ride (distro version)

Read os_version from yaml to over-ride teuthology defaults.

Signed-off-by: Sandon Van Ness <a38d61b815b8053a15c463c80449b5c080334ec7@inktank.com>

michaelsevilla/teuthology,t-miyamae/teuthology,ceph/teuthology,yghannam/teuthology,zhouyuan/teuthology,ktdreyer/teuthology,SUSE/teuthology,tchaikov/teuthology,dmick/teuthology,dmick/teuthology,dmick/teuthology,ivotron/teuthology,ivotron/teuthology,robbat2/teuthology,caibo2014/teuthology,caibo2014/teuthology,dreamhost/teuthology,t-miyamae/teuthology,ceph/teuthology,SUSE/teuthology,yghannam/teuthology,robbat2/teuthology,SUSE/teuthology,zhouyuan/teuthology,michaelsevilla/teuthology,ktdreyer/teuthology,tchaikov/teuthology,dreamhost/teuthology