Add a useful error message in case of an invalid hostname
Add a useful error message in case of an invalid hostname

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