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

* Remove support for python 2.6
* Remove skipsdist : Needs to be set only if 
  sdist is expensive
* Remove usedevelop : only needed when skipsdist 
  is set to True
* 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: I0441d771f554c86f3069bf62999c41f9b3ced99d
openstack/oslo.i18n