ms_srs: add config scheme to select NAS log level

we've provided only all_log_level so far but sometimes it's needed
to select the level per layer. This patch adds the ability
to do so for the NAS layer in the UE.

Change-Id: Iab2bce65e8af81f6d344849c97952e6441cb2846
diff --git a/src/osmo_gsm_tester/obj/ms_srs.py b/src/osmo_gsm_tester/obj/ms_srs.py
index aaeeca5..df83c2a 100644
--- a/src/osmo_gsm_tester/obj/ms_srs.py
+++ b/src/osmo_gsm_tester/obj/ms_srs.py
@@ -50,6 +50,7 @@
     config_schema = {
         'enable_pcap': schema.BOOL_STR,
         'log_all_level': schema.STR,
+        'log_nas_level': schema.STR
         }
     schema.register_config_schema('modem', config_schema)