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: I5b20c57ef760e7cacf5c657668c0959efd0deac3

openstack/openstack-ansible-galera_client,openstack/openstack-ansible-galera_client