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


Change-Id: If054a295dc58636bd317491e0f6cd454d1821a3e
openstack/tooz,openstack/tooz