Fix test errors on el6

Include full new-style implementation of assertRaises so that even old
versions of python can run all tests

Change-Id: I92e1f6a9c8931982b2467ca4b69a6ec819eb0d16
Signed-off-by: Saggi Mizrahi <smizrahi@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/11350
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>