tempest py26 setup

Simplifying the steps between devstack install and tempest run, by
 * installing tox<1.7, bacuse 1.7 is not compatible with our tox.ini
 * installing and patching 'discover' when the python version is 2.6

Change-Id: I8008837d2fae6cebeeeb051b63066279ca477e01
Merge branch 'master' of github.com:openstack-dev/devstack