Upgrade tox to 1.6
Upgrade tox to 1.6

tox 1.6 was released, which means that we can now take advantage of the
feature we added to it - which is using setup.py develop to install the
code into the virtualenv. The logic was taken from run_tests.sh - so the
performance issues around using tox vs. using install_venv should now be
gone.

Change-Id: Ib011a9add595696f5db5fe596db9d3d60f46d038
Fixes-Bug: #1233213

rdo-management/ironic,pshchelo/ironic,supermari0/ironic,devananda/ironic,citrix-openstack-build/ironic,naterh/ironic,redhat-openstack/ironic,bacaldwell/ironic,bacaldwell/ironic,ionutbalutoiu/ironic,openstack/ironic,SauloAislan/ironic,rackerlabs/ironic,dims/ironic,ionutbalutoiu/ironic,hpproliant/ironic,ramineni/myironic,varunarya10/ironic,JioCloud/ironic,naototty/vagrant-lxc-ironic,dims/ironic,openstack/ironic,froyobin/ironic,Tehsmash/ironic,naototty/vagrant-lxc-ironic,SauloAislan/ironic,citrix-openstack-build/ironic,pshchelo/ironic,NaohiroTamura/ironic,Tan0/ironic,debayanray/ironic_backup,faizan-barmawer/openstack_ironic,NaohiroTamura/ironic