ms: Allocate default msisdn internally

Don't wait until adding it to the subscriber data base, since the msisdn
may be needed beforehand (for instance in follow up patch to be passed
to osmo-msc configuration to route emergency calls).

Change-Id: I0d6902687e4beb3d6cdcefd4e343f21496100504
diff --git a/src/osmo_gsm_tester/obj/epc_amarisoft.py b/src/osmo_gsm_tester/obj/epc_amarisoft.py
index 3e91b5c..1291891 100644
--- a/src/osmo_gsm_tester/obj/epc_amarisoft.py
+++ b/src/osmo_gsm_tester/obj/epc_amarisoft.py
@@ -168,8 +168,7 @@
 
     def subscriber_add(self, modem, msisdn=None, algo_str=None):
         if msisdn is None:
-            msisdn = self.testenv.msisdn()
-        modem.set_msisdn(msisdn)
+            msisdn = modem.msisdn()
 
         if algo_str is None:
             algo_str = modem.auth_algo() or util.OSMO_AUTH_ALGO_NONE