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
* sitepackages : default is already false


Change-Id: I12de2f2175fce3a0b24c16a1fc55100c2927883f

tox.ini

@@ -1,17 +1,10 @@
 [tox]
 minversion = 1.6
-envlist = py26,py27,py34,pep8,pip-missing-reqs
-# NOTE(dhellmann): We cannot set skipdist=True
-# for oslo libraries because of the namespace package.
-#skipsdist = True
+envlist = py27,py34,pep8,pip-missing-reqs
 
 [testenv]
-# NOTE(dhellmann): We cannot set usedevelop=True
-# for oslo libraries because of the namespace package.
-#usedevelop = True
 whitelist_externals = bash
                       env
-install_command = pip install -U {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
 deps = .[fixtures]