Add choice introspection store_data backend

Add a data_store config for introspection store_data choice.

Related patch in ironic-inspector.[1]
[1] https://review.opendev.org/#/c/657770/

Change-Id: Ia244eb81f90c4708b2d0e758bc53f1c975e9acf8
diff --git a/ironic_tempest_plugin/config.py b/ironic_tempest_plugin/config.py
index a3b5019..819051e 100644
--- a/ironic_tempest_plugin/config.py
+++ b/ironic_tempest_plugin/config.py
@@ -198,4 +198,7 @@
     cfg.StrOpt('auto_discovery_target_driver',
                help="The driver to set on the newly discovered nodes. "
                     "Only has effect with auto_discovery_feature is True."),
+    cfg.StrOpt('data_store',
+               default='swift',
+               help="The storage backend for storing introspection data."),
 ]