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: I825550ecc5a3a15b981a177300d152f23a6541e9
Closes-Bug:  #1801467

openstack/networking-l2gw,openstack/networking-l2gw