Add support to change the status header for gatekeeper-ng.

mailer.py

Index: mailer.py
diff --git a/mailer.py b/mailer.py
index 4b529e08fccce81cc7fc169c0f814031db32ee72..4a60d81b25cfec0d78438092ce2307f43da74533 100644
--- a/mailer.py
+++ b/mailer.py
@@ -168,12 +168,14 @@ class Email(BaseHandler):
         app_identity.get_application_id())
 
     subject_template = build_data.get('subject_template')
+    status_header = build_data.get('status_header')
 
     template = gatekeeper_mailer.MailTemplate(build_data['waterfall_url'],
                                               build_data['build_url'],
                                               build_data['project_name'],
                                               from_addr,
-                                              subject=subject_template)
+                                              subject=subject_template,
+                                              status=status_header)
 
 
     text_content, html_content, subject = template.genMessageContent(build_data)