Update tox.ini to use new features
Update 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: Ief0a44a8d982a12ff5158c1223487d27b75b950e

fabian4/trove,changsimon/trove,redhat-openstack/trove,zhangg/trove,openstack/trove,cp16net/trove,redhat-openstack/trove,mmasaki/trove,fabian4/trove,denismakogon/trove-guestagent,redhat-openstack/trove,hplustree/trove,openstack/trove,zhangg/trove,zhujzhuo/openstack-trove,mmasaki/trove,cp16net/trove,denismakogon/trove-guestagent,changsimon/trove,zhujzhuo/openstack-trove,fabian4/trove,mmasaki/trove,changsimon/trove,cp16net/trove,hplustree/trove,zhujzhuo/openstack-trove