TEMPEST_CONFIG_DIR needs to be set for the api job to execute
successfully. Previously this was being set by the gate but local
execution required manually setting the variable.
Change-Id: Ic1364388e812a5fca26f37049242f7d09ee75fef
[testenv:api]
setenv = OS_TEST_PATH=./neutron/tests/api
+ TEMPEST_CONFIG_DIR={env:TEMPEST_CONFIG_DIR:/opt/stack/tempest/etc}
[testenv:functional]
setenv = OS_TEST_PATH=./neutron/tests/functional