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
diff --git a/tempest/config.py b/tempest/config.py
index cc0ba34..0834485 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -1217,6 +1217,11 @@
 
     def set_config_path(self, path):
         self._path = path
+        # FIXME(masayukig): bug#1783751 To pass the config file path to child
+        # processes, we need to set the environment variables here as a
+        # workaround.
+        os.environ['TEMPEST_CONFIG_DIR'] = os.path.dirname(path)
+        os.environ['TEMPEST_CONFIG'] = os.path.basename(path)
 
 
 CONF = TempestConfigProxy()