add support to override refspec of repos

This patch adds the top-level `overrides` entry, which is used to
override (or pin) the refspec of repositories. The main difference to a
direct override is that this logic only applies to repos that are
already defined. By that, a superset of all repos can be added to this
entry (similar to a global lockfile), but only the currently active ones
are affected. A new top-level keyword is required because everything
below the "repos" keyword is potentially defined by "default" values.
For the locking mechanism, a clear separation between overrides (only
override if existing) and definitions is required to be able to define a
global lockfile with all possible repos, while just defining some repos.

Proposed-by: Ross Burton <ross@burtonini.com>
Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
[Jan: also bump __file_version__]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/schema-kas.json

@@ -89,6 +89,24 @@
                 }
             }
         },
+        "overrides": {
+            "type": "object",
+            "additionalProperties": false,
+            "properties": {
+                "repos": {
+                    "type": "object",
+                    "additionalProperties": {
+                        "type": "object",
+                        "additionalProperties": false,
+                        "properties": {
+                            "refspec" : {
+                                "type": "string"
+                            }
+                        }
+                    }
+                }
+            }
+        },
         "machine": {
             "type": "string"
         },