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_enb.py b/src/osmo_gsm_tester/srs_enb.py
index 15c1e0c..6f47e75 100644
--- a/src/osmo_gsm_tester/srs_enb.py
+++ b/src/osmo_gsm_tester/srs_enb.py
@@ -187,7 +187,7 @@
             f.write(r)
 
     def configure(self):
-        values = super().configure('srsenb')
+        values = super().configure(['srsenb'])
 
         # Convert parsed boolean string to Python boolean:
         self.enable_pcap = util.str2bool(values['enb'].get('enable_pcap', 'false'))