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: Id9eaa2cb2d9aa6648d6ac345029cebae96f9b37e
Closes-Bug:  #1801683

openstack/python-muranoclient,openstack/python-muranoclient