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: Iac1ea510102b89d1bc53b10aa2f53ed673abb014

os-cloud/openstack-ansible-pip_install,openstack/openstack-ansible-pip_install,openstack/openstack-ansible-pip_install,os-cloud/openstack-ansible-pip_install