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

tox.ini

@@ -1,9 +1,10 @@
 [tox]
 minversion = 2.0
 skipsdist = True
-envlist = pep8,py27,py37
+envlist = pep8,py37
 
 [testenv]
+basepython = python3
 usedevelop = True
 install_command = pip install {opts} {packages}
 deps =
@@ -13,15 +14,10 @@ deps =
 commands=
     python setup.py testr --slowest --testr-args='{posargs}'
 
-[testenv:py26]
-commands =
-
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
 [testenv:pep8]
-basepython = python3
 commands = ./run-flake8
 
 [flake8]
@@ -30,5 +26,4 @@ exclude = .venv,.tox,dist,*.egg,build,releasenotes
 show-source = true
 
 [testenv:releasenotes]
-basepython = python3
 commands = bash -c tools/releasenotes_tox.sh