Fix last_reachable timestamp bug

Change-Id: I0d479c9aef59866db164790247eca2c3c6e1f0bf
diff --git a/prom.py b/prom.py
index 9a517e1..5c4668c 100644
--- a/prom.py
+++ b/prom.py
@@ -58,11 +58,9 @@
             metric = elem['metric']
             if 'imsi' in metric and 'mobile_ip' in metric:
                 imsi = metric['imsi']
-                if imsi in devices:
-                    # log.info("{} added to list of monitored devices".format(imsi))
-                    devices[imsi].ip = metric['mobile_ip']
-                else:
-                    log.error("Ignoring device as it was not reported by ROC - {}".format(metric))
+                for k in devices:
+                    if devices[k].imsi == imsi:
+                        devices[k].ip = metric['mobile_ip']
             else:
                 log.error("Ignoring device as imsi or mobile-ip not found - {}".format(metric))