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: I27348462a21daab479d76b24b48e3a4e017504cc

zhimin711/nova,Juniper/nova,hanlind/nova,phenoxim/nova,bigswitch/nova,mahak/nova,openstack/nova,vmturbo/nova,rahulunair/nova,hanlind/nova,cyx1231st/nova,sebrandon1/nova,openstack/nova,klmitch/nova,BeyondTheClouds/nova,jianghuaw/nova,NeCTAR-RC/nova,cloudbase/nova,dims/nova,mikalstill/nova,BeyondTheClouds/nova,alaski/nova,cernops/nova,gooddata/openstack-nova,gooddata/openstack-nova,sebrandon1/nova,rajalokan/nova,cloudbase/nova,CEG-FYP-OpenStack/scheduler,jianghuaw/nova,rajalokan/nova,klmitch/nova,klmitch/nova,cloudbase/nova,NeCTAR-RC/nova,vmturbo/nova,mikalstill/nova,jianghuaw/nova,cyx1231st/nova,vmturbo/nova,rajalokan/nova,Juniper/nova,gooddata/openstack-nova,rajalokan/nova,cernops/nova,rahulunair/nova,sebrandon1/nova,alaski/nova,rahulunair/nova,jianghuaw/nova,Juniper/nova,gooddata/openstack-nova,CEG-FYP-OpenStack/scheduler,Juniper/nova,cernops/nova,phenoxim/nova,hanlind/nova,mahak/nova,BeyondTheClouds/nova,openstack/nova,dims/nova,mahak/nova,zhimin711/nova,mikalstill/nova,vmturbo/nova,bigswitch/nova,klmitch/nova