remove py27 in tox.ini from tripleo-image-elements
remove py27 in tox.ini from tripleo-image-elements

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: I2661314199ea252d080dfff2b5daa5d3b87ccabb

openstack/tripleo-image-elements,openstack/tripleo-image-elements