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/amarisoft_epc.py b/src/osmo_gsm_tester/amarisoft_epc.py
index 0361f54..eb9be94 100644
--- a/src/osmo_gsm_tester/amarisoft_epc.py
+++ b/src/osmo_gsm_tester/amarisoft_epc.py
@@ -141,7 +141,7 @@
             self.remote_log_file = remote_run_dir.child(AmarisoftEPC.LOGFILE)
             self.remote_ifup_file = remote_run_dir.child(AmarisoftEPC.IFUPFILE)
 
-        values = super().configure('amarisoftepc')
+        values = super().configure(['amarisoft', 'amarisoftepc'])
 
         logfile = self.log_file if self._run_node.is_local() else self.remote_log_file
         ifupfile = self.ifup_file if self._run_node.is_local() else self.remote_ifup_file