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: I04ff8e8cce4723e7df0577388b4a8011faeefdf1

tox.ini

@@ -136,11 +136,9 @@ deps =
 commands =
     {[testenv:ansible]commands}
     ansible-playbook -i {toxinidir}/tests/inventory \
-                     -e "install_test_packages=True" \
                      {toxinidir}/tests/test.yml -vvvv
     # Idempotency test
     bash -c 'ansible-playbook -i {toxinidir}/tests/inventory \
-                     -e "install_test_packages=True" \
                      {toxinidir}/tests/test.yml \
                      | grep -q "changed=0.*failed=0" \
                      && (echo "Idempotence test: pass" && exit 0) \