Make amarisoft license server optional

* Since license_server_addr logic is generic among all amarisoft
  software, let's move it to its own config category "amarisoft", which
  all amarisoft classes (amarisoft_{enb,epc}.py as of now) will import
  during configure().
* The license server is disabled by default (value 0.0.0.0) since the
  usual case is to have a license in the local system.
* the license server can be enabled using scenario
  cfg-amarisoft-license@A.B.C.D where A.B.C.D is the address to use.

Change-Id: Id5456ad9497e91370857b80ff9a389111d3611ff
diff --git a/src/osmo_gsm_tester/srs_epc.py b/src/osmo_gsm_tester/srs_epc.py
index 708ca5b..b2d728a 100644
--- a/src/osmo_gsm_tester/srs_epc.py
+++ b/src/osmo_gsm_tester/srs_epc.py
@@ -158,7 +158,7 @@
         self.pcap_file = self.run_dir.child(srsEPC.PCAPFILE)
         self.dbg(config_file=self.config_file, db_file=self.db_file)
 
-        values = super().configure('srsepc')
+        values = super().configure(['srsepc'])
 
         # Convert parsed boolean string to Python boolean:
         self.enable_pcap = util.str2bool(values['epc'].get('enable_pcap', 'false'))