Use old python as "basepython" for tox.

Thanks to
https://git.openstack.org/cgit/stackforge/fuel-web/commit/?id=02157968d34f1a4c5557d45560f104db312ea7e9
Allow the --cut (-u) option to be specified twice, once for each read end.

This is based on a patch submitted by Rasmus Borup Hansen in issue #88. Thanks!