ms: add optional param to specify fixed APN IP address assigned to a MS

this allows to specify a fixed APN address for each subscriber in the
HLR. If the param is not provided in the resources config, the default
allocation type is used, which is to dynamically allocate an IP in the
supported range.

Change-Id: Ie6fcd7987dc7bc62a686bee3a10c67e3dd813419
diff --git a/src/osmo_gsm_tester/resource.py b/src/osmo_gsm_tester/resource.py
index 757a9c5..5862aef 100644
--- a/src/osmo_gsm_tester/resource.py
+++ b/src/osmo_gsm_tester/resource.py
@@ -100,6 +100,7 @@
         'modem[].imsi': schema.IMSI,
         'modem[].ki': schema.KI,
         'modem[].auth_algo': schema.AUTH_ALGO,
+        'modem[].apn_ipaddr': schema.IPV4,
         'modem[].remote_user': schema.STR,
         'modem[].addr': schema.IPV4,
         'modem[].ciphers[]': schema.CIPHER,