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: I1aa87d5ddfaefb1365c6eb42dca413ada2b71a35

openstack/openstack-ansible-rabbitmq_server,openstack/openstack-ansible-rabbitmq_server