Make random extension range configurable

Previously if subscriber was automatically created it got assigned
random MSISDN number between 20000 and 49999. Make it configurable with
new vty command "subscriber-create-on-demand random" and expand vty
tests to check it.

Change-Id: I040a1d227b0c7a1601dc7c33eccb0007941408a6
Related: OS#1658
diff --git a/openbsc/src/libmsc/gsm_04_08.c b/openbsc/src/libmsc/gsm_04_08.c
index 92c4cfe..6704497 100644
--- a/openbsc/src/libmsc/gsm_04_08.c
+++ b/openbsc/src/libmsc/gsm_04_08.c
@@ -537,7 +537,8 @@
 		if (!subscr_regexp_check(net, imsi))
 			return NULL;
 
-	return subscr_create_subscriber(net->subscr_group, imsi);
+	return subscr_create_subscriber(net->subscr_group, imsi, net->ext_min,
+					net->ext_max);
 }
 
 /* Parse Chapter 9.2.11 Identity Response */