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: I30aa29a41d4a2084cb9d23e9dfab80d2c5cea78c

openstack/networking-arista,stackforge/networking-arista,openstack/networking-arista