Fix existing_mailbox check

cloud/webfaction/webfaction_mailbox.py

@@ -99,7 +99,7 @@ def main():
         module.params['login_password']
     )
 
-    mailbox_list = webfaction.list_mailboxes(session_id)
+    mailbox_list = [x['mailbox'] for x in webfaction.list_mailboxes(session_id)]
     existing_mailbox = mailbox_name in mailbox_list
 
     result = {}