Create subscribers on demand

Add a new vty option and allow to optionally generate a random msisdn,
as well as setting the default NAM:

subscriber-create-on-demand (no-msisdn|<3-15>) (none|cs|ps|both)

Thanks to Vadim for the random MSISDN patch [1], which was squashed into
this one.

[1] Change-Id: I475c71f9902950fa7498855a616e1ec231fad6ac

Depends on: Idc74f4d94ad44b9fc1b6d43178f5f33d551ebfb1 (libosmocore)
Change-Id: I0c9fe93f5c24b5e9fefb513c4d049fb7ebd47ecd
Related: OS#2542
diff --git a/tests/test_nodes.vty b/tests/test_nodes.vty
index a4e53db..bf2afaf 100644
--- a/tests/test_nodes.vty
+++ b/tests/test_nodes.vty
@@ -86,6 +86,8 @@
   ncss-guard-timeout <0-255>
   store-imei
   no store-imei
+  subscriber-create-on-demand (no-msisdn|<3-15>) (none|cs|ps|cs+ps)
+  no subscriber-create-on-demand
 
 OsmoHLR(config-hlr)# gsup
 OsmoHLR(config-hlr-gsup)# list