format to_addr on post request so that it allows dynamic values from …

…matched field

Discussion regarding this can be found here. #1561

elastalert/alerters/opsgenie.py

@@ -126,7 +126,7 @@ def alert(self, matches):
         proxies = {'https': self.opsgenie_proxy} if self.opsgenie_proxy else None
 
         try:
-            r = requests.post(self.to_addr, json=post, headers=headers, proxies=proxies)
+            r = requests.post(self.to_addr.format(**matches[0]), json=post, headers=headers, proxies=proxies)
 
             elastalert_logger.debug('request response: {0}'.format(r))
             if r.status_code != 202: