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/example/defaults.conf b/example/defaults.conf
index 56a41e5..aa1e1dc 100644
--- a/example/defaults.conf
+++ b/example/defaults.conf
@@ -101,8 +101,8 @@
   rlc_drb_mode: UM
   enable_pcap: false
 
-amarisoftepc:
-  license_server_addr: 10.12.1.139
+amarisoft:
+  license_server_addr: 0.0.0.0
 
 enb:
   mcc: 901
@@ -115,7 +115,6 @@
   enable_pcap: false
 
 amarisoftenb:
-  license_server_addr: 10.12.1.139
   num_prb: 100
   enable_pcap: false