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/templates/amarisoft_enb.cfg.tmpl b/src/osmo_gsm_tester/templates/amarisoft_enb.cfg.tmpl
index 73b4fd2..fab1649 100644
--- a/src/osmo_gsm_tester/templates/amarisoft_enb.cfg.tmpl
+++ b/src/osmo_gsm_tester/templates/amarisoft_enb.cfg.tmpl
@@ -3,11 +3,12 @@
  */
 {
 
+%if enb.license_server_addr != '0.0.0.0':
   license_server: {
     server_addr: "${enb.license_server_addr}",
     name: "amarisoft",
   },
-
+%endif
 
   /* Log filter: syntax: layer.field=value[,...]