Optimise the tox_install script
Optimise the tox_install script

This change prevents unnessesary reinstalls of the same version of
neutron when running the openstack CI which should speed things up.

Change-Id: I8ccd2fedb13f6a4c99d03dc7b3822ef5d8959cee

Tehsmash/networking-cisco,noironetworks/networking-cisco,noironetworks/networking-cisco,Tehsmash/networking-cisco,openstack/networking-cisco,openstack/networking-cisco