Add a configurable fallback URL for emails

This will allow the fixing of an issue where changes created
by things using the API directly, such as the Gerrit plugin,
cause broken URLs in notification emails.

Change-Id: Ibad10fa35b7acaaa59cbb28b0960571480f80552

storyboard/plugin/email/__init__.py

@@ -33,6 +33,9 @@ PLUGIN_OPTS = [
     cfg.StrOpt("reply_to",
                default=None,
                help="The email address of the Reply-To header (optional)."),
+    cfg.StrOpt("default_url",
+               default=None,
+               help="The default/fallback url base to use in emails."),
     cfg.StrOpt("smtp_host",
                default='localhost',
                help="The SMTP server to use."),

storyboard/plugin/email/workers.py

@@ -282,6 +282,10 @@ class SubscriptionEmailWorker(EmailWorkerBase):
         if email_config.reply_to:
             factory.add_header('Reply-To', email_config.reply_to)
 
+        # If there is a fallback URL configured, use it if needed
+        if email_config.default_url and url is None:
+            url = email_config.default_url
+
         # Resolve the resource instance
         resource_instance = self.resolve_resource_by_name(session, resource,
                                                           resource_id)