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/templates/srsue.conf.tmpl b/src/osmo_gsm_tester/templates/srsue.conf.tmpl
index 0939823..272ae48 100644
--- a/src/osmo_gsm_tester/templates/srsue.conf.tmpl
+++ b/src/osmo_gsm_tester/templates/srsue.conf.tmpl
@@ -109,6 +109,7 @@
 all_hex_limit = 32
 filename = ${ue.log_filename}
 file_max_size = -1
+nas_level = ${ue.log_nas_level}
 
 #####################################################################
 # USIM configuration