software/slapos-master: add required software.cfg.json for client compatibility

software/slapos-master/software.cfg.json

+{
+  "name": "SlapOS Master",
+  "description": "SlapOS Master",
+  "serialisation": "json-in-xml",
+  "software-type": {
+    "default": {
+      "title": "Default",
+      "software-type": "default",
+      "request": "../erp5/instance-erp5-input-schema.json",
+      "response": "../erp5/instance-erp5-output-schema.json",
+      "index": 0
+    }
+  }
+}