stevedore 1.11.0 does not support python2.6

Python2.6 support was dropped in 1.10.0 release of stevedore and workaround for python2.6 was removed in release 1.11.0 (http://git.openstack.org/cgit/openstack/stevedore/commit/?id=72e3e1177dc23b4f3d358294d68b58c01d7c5931).

Therefore the support for python2.6 should also dropped for aminator or workaround should be implemented.
