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: I5c7046ab40697263f9399f38f2af7bf5dddad879
Closes-Bug:  #1801681

openstack/python-mistralclient,openstack/python-mistralclient