Update min tox version to 2.0
Update min tox version to 2.0

The commands used by constraints need at least tox 2.0.  Update to
reflect reality, which should help with local running of constraints
targets.

Change-Id: Ife30a7c189783b264f5df9b7bacd0a92df2fec2a

openstack/python-brick-cinderclient-ext,openstack/python-brick-cinderclient-ext