schema.json: Fix required property constraints and provide default values

Also, describe a bit more "serialisation" override mechanism: these
properties cannot be made required at schema level, but for any
software-type entry a serialisation must be provided.

schema.json

@@ -2,49 +2,56 @@
   "$schema": "http://json-schema.org/draft-04/schema#",
   "description": "Slapos Software Release instantiation descriptor",
   "additionalProperties": false,
+  "required": [
+    "software-type"
+  ],
   "properties": {
     "name": {
       "description": "A short human-friendly name for the sofware release",
+      "default": "",
       "type": "string"
     },
     "description": {
       "description": "A short description of the sofware release",
+      "default": "",
       "type": "string"
     },
     "serialisation": {
       "description": "How the parameters and results are serialised",
-      "require": true,
       "enum": ["xml", "json-in-xml"],
       "type": "string"
     },
     "software-type": {
       "description": "Existing software types",
-      "require": true,
       "patternProperties": {
         ".*": {
           "description": "Software type declaration",
           "additionalProperties": false,
+          "required": [
+            "request",
+            "response"
+          ],
           "properties": {
             "title": {
               "description": "A human-friendly title of the software type",
+              "default": "",
               "type": "string"
             },
             "description": {
               "description": "A human-friendly description of the software type",
+              "default": "",
               "type": "string"
             },
             "serialisation": {
-              "description": "How the parameters and results are serialised, if different from global setting",
+              "description": "How the parameters and results are serialised, if different from global setting, required if global setting is not provided",
               "enum": ["xml", "json-in-xml"],
               "type": "string"
             },
             "request": {
-              "require": true,
               "description": "URL, relative to Software Release base path, of a json schema for values expected by instance of current software type",
               "type": "string"
             },
             "response": {
-              "require": true,
               "description": "URL, relative to Software Release base path, of a json schema for values published by instance of current software type",
               "type": "string"
             },
@@ -54,10 +61,12 @@
             },
             "shared" : {
               "description": "Define if the request will request a Slave or Software Instance.",
+              "default": "false",
               "type": "boolean"
             },
             "index": {
               "description": "Value to use instead of software type id to sort them (in order to display most relevant software types earlier in a list, for example)",
+              "default": 0,
               "type": "number"
             }
           },