stages/org.osbuild.ostree.config: support bls-append-except-default

Support setting the sysroot.bls-append-except-default value in the
OSTree config. This is used by CoreOS to support configuration used
for GRUB password support https://github.com/coreos/fedora-coreos-tracker/issues/1333

test/data/manifests/fedora-coreos-container.json

@@ -466,7 +466,8 @@
             "config": {
               "sysroot": {
                 "readonly": false,
-                "bootloader": "none"
+                "bootloader": "none",
+                "bls-append-except-default": "grub_users=\"\""
               }
             }
           }

test/data/manifests/fedora-coreos-container.mpp.yaml

@@ -89,6 +89,7 @@ pipelines:
             sysroot:
               readonly: false
               bootloader: none
+              bls-append-except-default: grub_users=""
       - type: org.osbuild.mkdir
         options:
           paths: