Revert "Revert "Revert "Remove TEMPEST_CONFIG_DIR in the api tox env"""

Revert "Revert "Revert "Remove TEMPEST_CONFIG_DIR in the api tox env"""

This reverts commit be63289c273fa02159a6958e0b456d5fda217431.

Yes, it's a third order revert. Ain't it cool?

Now that tox 2.3.1 is used in gate, and it is expected to fix the issue
that we originally hacked around here, let's revert to the previous
code.

For the least, it makes API target executable locally without the need
to set TEMPEST_CONFIG_DIR in the external environment.

Also bump minimal tox version to 2.3 to avoid users hitting the old
issue with their tox 2.2.* package.

Change-Id: Ic64cb466d117ad39436cb1d079c8f5657260da2c
Related-Bug: #1515335