Remove install_test_packages variable
Remove install_test_packages variable

With https://review.openstack.org/381479 merged, setting this
variable in tox.ini via an extra var is unnecessary.

Change-Id: Ie2cc718173015a681209d73737a4a7b60af7216c

juledwar/openstack-ansible-os_ironic,juledwar/openstack-ansible-os_ironic