vty: enable show subscribers filtered by IMEI

In 89fda3024a0c9a422cc5e5033b016ce0ab6c1ff3 I added a
vty command to show a summary of filtered subscribers by imsi
or msisdn. In practice there is a also need to be able to
filter on IMEI.

The idea here is not to replace every operation that could be
done directly on the sql database in the vty, but this one
is useful.

Change-Id: Ic4a11d3ebcf8909c68e9f4617e94dc822491e008
diff --git a/tests/test_nodes.vty b/tests/test_nodes.vty
index cce5703..bebe9a0 100644
--- a/tests/test_nodes.vty
+++ b/tests/test_nodes.vty
@@ -14,7 +14,7 @@
 ...
   show gsup-connections
   show subscribers all
-  show subscribers (imsi|msisdn) FILTER
+  show subscribers (imei|imsi|msisdn) FILTER
   show subscribers (cs|ps) (on|off)
   show subscribers last-seen
   subscriber (imsi|msisdn|id|imei) IDENT show