Return missing 'return' in build_option_parser

It was removed by mistake and should be removed
back.

Change-Id: I0963c2d63b54d1bf6f75e703038712f4632803c8

glareclient/osc/plugin.py

@@ -51,3 +51,5 @@ def build_option_parser(parser):
         default=utils.env('OS_ARTIFACT_API_VERSION'),
         help=_('Artifact API version, default=%s '
                '(Env: OS_ARTIFACT_API_VERSION)') % DEFAULT_API_VERSION)
+
+    return parser