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_enb.py b/src/osmo_gsm_tester/amarisoft_enb.py
index 0890f90..79b90a7 100644
--- a/src/osmo_gsm_tester/amarisoft_enb.py
+++ b/src/osmo_gsm_tester/amarisoft_enb.py
@@ -159,7 +159,7 @@
             self.remote_config_drb_file = remote_run_dir.child(AmarisoftENB.CFGFILE_DRB)
             self.remote_log_file = remote_run_dir.child(AmarisoftENB.LOGFILE)
 
-        values = super().configure('amarisoftenb')
+        values = super().configure(['amarisoft', 'amarisoftenb'])
         self._num_cells = int(values['enb'].get('num_cells', None))
         assert self._num_cells