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/ctrl_commands.c b/openbsc/src/libmsc/ctrl_commands.c
index 0d6a37c..a02db36 100644
--- a/openbsc/src/libmsc/ctrl_commands.c
+++ b/openbsc/src/libmsc/ctrl_commands.c
@@ -18,6 +18,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  */
+
 #include <osmocom/ctrl/control_cmd.h>
 #include <openbsc/gsm_data.h>
 #include <openbsc/gsm_subscriber.h>
@@ -95,7 +96,9 @@
 
 	subscr = subscr_get_by_imsi(net->subscr_group, imsi);
 	if (!subscr)
-		subscr = subscr_create_subscriber(net->subscr_group, imsi);
+		subscr = subscr_create_subscriber(net->subscr_group, imsi,
+						  net->ext_min,
+						  net->ext_max);
 	if (!subscr)
 		goto fail;