Fix to enable `--config-file` option
Fix to enable `--config-file` option

This commit sets the `--config-file` path to the environment variables
`TEMPEST_CONFIG_DIR` and `TEMPEST_CONFIG` to enable the option as a
workaround.
The `path` is passed to `set_config_path()` however, the running test
process is a child process which is a different instance.

Change-Id: I8a65ee0f82e41f9659b3f40e2685c0ee0f290342
Related-Bug: #1783751

masayukig/tempest,masayukig/tempest,cisco-openstack/tempest,openstack/tempest,openstack/tempest,cisco-openstack/tempest