mounts: change schema meta information

Define the mount schema in the actual mounts at a higher level. This
is in preparation to give the modules more control over the `source`
and `target` properties.

mounts/org.osbuild.btrfs

@@ -13,8 +13,23 @@ from typing import Dict
 from osbuild import mounts
 
 
-SCHEMA = """
-"additionalProperties": False
+SCHEMA_2 = """
+"additionalProperties": false,
+"required": ["name", "type", "source", "target"],
+"properties": {
+  "name": { "type": "string" },
+  "type": { "type": "string" },
+  "source": {
+    "type": "string"
+  },
+  "target": {
+    "type": "string"
+  },
+  "options": {
+    "type": "object",
+    "additionalProperties": true
+  }
+}
 """