Remove python 2.6 and cleanup tox.ini
Remove python 2.6 and cleanup tox.ini

* Remove support for python 2.6
* Remove install_command : We can just use the 
  default, we don't need to override
* Remove setenv : We can just use the default as
  we don't need any extra environment variables
* Remove requirements.txt from deps, as this is
  already added automatically

Change-Id: I4f650eaa07624b889a52570e628a21e905f2bd46
openstack/oslotest,openstack/oslotest