Make sure that tox uses python2 by default

On Ubuntu 14.10 the tox package creates python 3 virtual environments by
default. Since our tools are not python 3 compatible, we need to force
python 2 virtual environments.

Change-Id: Id2c2129f82b3128d020db5c6500b4b7443718d13
Merge "Make sure that tox uses python2 by default"