Fix package name in metadata

Correct package name for Cinder Tempest plugin
is 'cinder-tempest-plugin'. It's used to upload package to
PyPI.

Change-Id: I6f35ec536031c02b9e6283311e25f7b78ee9c677
diff --git a/setup.cfg b/setup.cfg
index cb45891..1ae93ad 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,5 +1,5 @@
 [metadata]
-name = cinder_tempest_plugin
+name = cinder-tempest-plugin
 summary = Tempest plugin tests for Cinder.
 description-file =
     README.rst