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/epc.py b/src/osmo_gsm_tester/epc.py
index da8302c..6aa17e7 100644
--- a/src/osmo_gsm_tester/epc.py
+++ b/src/osmo_gsm_tester/epc.py
@@ -33,10 +33,13 @@
         self.suite_run = suite_run
         self._run_node = run_node
 
-    def configure(self, default_specifics):
+    def configure(self, config_specifics_li):
         values = dict(epc=config.get_defaults('epc'))
-        config.overlay(values, dict(epc=config.get_defaults(default_specifics)))
+        for config_specifics in config_specifics_li:
+            config.overlay(values, dict(epc=config.get_defaults(config_specifics)))
         config.overlay(values, dict(epc=self.suite_run.config().get('epc', {})))
+        for config_specifics in config_specifics_li:
+            config.overlay(values, dict(epc=self.suite_run.config().get(config_specifics, {})))
         config.overlay(values, dict(epc={'run_addr': self.addr()}))
         return values