jabber_mirror: Calculate the zulip domain from the client email address

We were previously calculating it from the zulip_email option, which might not
be set if it is being specified via the config file.

(imported from commit 76866c239ca63ef90a7967c9a6027aeec9be6390)

bots/jabber_mirror.py

@@ -389,10 +389,10 @@ option does not affect login credentials.'''.replace("\n", " "))
         sys.exit("You must specify your Jabber JID and Jabber password either "
                  + "in the Zulip configuration file or on the commandline")
 
-    # This won't work for open realms
-    options.zulip_domain = options.zulip_email.partition('@')[-1]
-
     zulip = ZulipToJabberBot(zulip.init_from_options(options, "JabberMirror/" + __version__))
+    # This won't work for open realms that don't have a consistent domain
+    options.zulip_domain = zulip.client.email.partition('@')[-1]
+
     try:
         jid = JID(options.jid)
     except InvalidJID as e: