Make sure tox is installed when setting up tempest

Partial backport of I8008837d2fae6cebeeeb051b63066279ca477e01

In order to make grenade easier to run outside of the gate make sure tox
is installed when tempest is set up. Currently one has to install tox
manually to get grenade to work.

Change-Id: Ie20de0c523cf1c31683fe69fa88f73705bed06b9
Merge "Make sure tox is installed when setting up tempest" into stable/icehouse