Add flag to generate tempest plugin list

This commit adds a flag to generate a tempest plugin list. In a local
environment, we don't need to generate a tempest plugin list, always
when we just need to see the other documents. Besides, generating the
tempest plugin list takes a long time. When GENERATE_TEMPEST_PLUGIN_LIST
environment variable is 'true'(default), the plugin list is created.
When the other value such as 'false' is specified, it isn't.

Change-Id: I6fd3cb3045ba8bbfb39c597413e13faa3cd24ae3
diff --git a/tox.ini b/tox.ini
index 2120818..6f37d00 100644
--- a/tox.ini
+++ b/tox.ini
@@ -20,7 +20,7 @@
     PYTHONWARNINGS=default::DeprecationWarning
     BRANCH_NAME=master
     CLIENT_NAME=tempest
-passenv = OS_STDOUT_CAPTURE OS_STDERR_CAPTURE OS_TEST_TIMEOUT OS_TEST_LOCK_PATH OS_TEST_PATH TEMPEST_CONFIG TEMPEST_CONFIG_DIR http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY ZUUL_CACHE_DIR REQUIREMENTS_PIP_LOCATION
+passenv = OS_STDOUT_CAPTURE OS_STDERR_CAPTURE OS_TEST_TIMEOUT OS_TEST_LOCK_PATH OS_TEST_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
 usedevelop = True
 install_command =
     {toxinidir}/tools/tox_install.sh {env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}