Updates tox.ini to use new features
Updates tox.ini to use new features

Reasons:
 - tox update v1.6

Changes:
 - tox 1.6 allows us to skip the sdist step, which is slow.
 - It also allows us to override the install line. In this case, it's
   important as it allows us to stop getting pre-release software we
   weren't asking for.

Original patch by Monty Taylor, talked about here:
http://lists.openstack.org/pipermail/openstack-dev/2013-September/015495.html

Change-Id: Idbf8e08383232ae0af5b245c1abe91a7f852bfcf

JioCloud/python-glanceclient,alexpilotti/python-glanceclient,klmitch/python-glanceclient,openstack/python-glanceclient,varunarya10/python-glanceclient,mmasaki/python-glanceclient,openstack/python-glanceclient,alexpilotti/python-glanceclient,mmasaki/python-glanceclient,JioCloud/python-glanceclient,klmitch/python-glanceclient,varunarya10/python-glanceclient