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/libbsc/net_init.c b/openbsc/src/libbsc/net_init.c
index afcaaf3..4636d57 100644
--- a/openbsc/src/libbsc/net_init.c
+++ b/openbsc/src/libbsc/net_init.c
@@ -104,7 +104,8 @@
 	net->stats.bts.rsl_fail = osmo_counter_alloc("net.bts.rsl_fail");
 
 	net->mncc_recv = mncc_recv;
-
+	net->ext_min = GSM_MIN_EXTEN;
+	net->ext_max = GSM_MAX_EXTEN;
 	gsm_net_update_ctype(net);
 
 	return net;