tox: pass TEMPEST_CONFIG_DIR envvar into api target environment

tox: pass TEMPEST_CONFIG_DIR envvar into api target environment

Currently, API job skips all tests because TEMPEST_CONFIG_DIR is not set
properly. The environment variable was hardcoded to
/opt/stack/tempest/etc before, and since that's the location for tempest
configuration files that was used in gate, it worked fine there.

But after we were forced to remove the hardcoded value from the file due
to regression in tox 2.2.0+ (If3742b6eabe295a5aaeb4901c55909a70c451698),
it was neither hardcoded, nor passed from the external gate environment.

Note that since tox 2.0.0, the tool does not pass all envvars into tox
targets, but require explicit specification of the list of variables
that should be allowed into the target environment, using passenv
directive.

So that's what the patch does: it allows TEMPEST_CONFIG_DIR to get into
api tox target, which should be enough to make the API job avoid
skipping all tests.

Users are still required to set TEMPEST_CONFIG_DIR variable locally if
they want to execute the suite locally.

Closes-Bug: #1518632
Change-Id: Ia7dcc6785390a6012c7ba16a14573aa44f2b6fb5