GABBI_TEMPEST_PATH in tox passenv and devstack playbook

The gabbi-tempest plugin uses an environment variable,
GABBI_TEMPEST_PATH, to identify directories in which to
find the gabbi [1] tests that will be run. This will be
used by a forthcoming zuul job [2] (hosted by the plugin)
that will automate gabbi-based service testing as described
in https://anticdent.org/gabbi-in-the-gate.html .

By setting the environment in the devstack-tempest playbook
we avoid needing to duplicate the playbook: we can use
it directly and have less risk of plays diverging. The
calling job (which doesn't allow the "environment" key)
sets a "var" which then sets the environment variable.

[1] https://gabbi.readthedocs.io
[2] https://review.openstack.org/611678

Needed-By: https://review.openstack.org/611678
Change-Id: I5aa890baa489f434b95d21e4e618f5202ac550e4

playbooks/devstack-tempest.yaml

@@ -7,6 +7,11 @@
 
 # We run tests only on one node, regardless how many nodes are in the system
 - hosts: tempest
+  environment:
+    # This enviroment variable is used by the optional tempest-gabbi
+    # job provided by the gabbi-tempest plugin. It can be safely ignored
+    # if that plugin is not being used.
+    GABBI_TEMPEST_PATH: "{{ gabbi_tempest_path }}"
   roles:
     - setup-tempest-run-dir
     - setup-tempest-data-dir

tox.ini

@@ -20,7 +20,7 @@ setenv =
     OS_STDERR_CAPTURE=1
     OS_TEST_TIMEOUT=160
     PYTHONWARNINGS=default::DeprecationWarning,ignore::DeprecationWarning:distutils,ignore::DeprecationWarning:site
-passenv = OS_STDOUT_CAPTURE OS_STDERR_CAPTURE OS_TEST_TIMEOUT OS_TEST_LOCK_PATH TEMPEST_CONFIG TEMPEST_CONFIG_DIR http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY ZUUL_CACHE_DIR REQUIREMENTS_PIP_LOCATION GENERATE_TEMPEST_PLUGIN_LIST
+passenv = OS_STDOUT_CAPTURE OS_STDERR_CAPTURE OS_TEST_TIMEOUT OS_TEST_LOCK_PATH TEMPEST_CONFIG TEMPEST_CONFIG_DIR http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY ZUUL_CACHE_DIR REQUIREMENTS_PIP_LOCATION GENERATE_TEMPEST_PLUGIN_LIST GABBI_TEMPEST_PATH
 usedevelop = True
 install_command = pip install {opts} {packages}
 whitelist_externals = *