remove py27 in tox.ini from tripleo-heat-templates
remove py27 in tox.ini from tripleo-heat-templates

Remove python 2 from envlist parameter. Check the link:

https://etherpad.openstack.org/p/drop-python2-support
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

The plan is drop the python 2 support from OpenStack in Ussuri release.
Move the basepython parameter.

Change-Id: I794fe6677328d74bef305dcfc15ab81038ae2b87

openstack/tripleo-heat-templates,openstack/tripleo-heat-templates