ctrl: Add a command to check how many bts are configured

This can be used to query how many bts are configured to
check if all of them are locked or not.
diff --git a/openbsc/tests/ctrl_test_runner.py b/openbsc/tests/ctrl_test_runner.py
index 502da76..160333d 100644
--- a/openbsc/tests/ctrl_test_runner.py
+++ b/openbsc/tests/ctrl_test_runner.py
@@ -453,6 +453,12 @@
     def ctrl_app(self):
         return (4249, "./src/osmo-nitb/osmo-nitb", "OsmoBSC", "nitb")
 
+    def testNumberOfBTS(self):
+        r = self.do_get('number-of-bts')
+        self.assertEquals(r['mtype'], 'GET_REPLY')
+        self.assertEquals(r['var'], 'number-of-bts')
+        self.assertEquals(r['value'], '1')
+
     def testSubscriberAddRemove(self):
         r = self.do_set('subscriber-modify-v1', '2620345,445566')
         self.assertEquals(r['mtype'], 'SET_REPLY')