Add x-kubernetes-preserve-unknown-fields for ignition

This allows MCC/MCD to validate the ignition section instead, much
like we've always done. Otherwise, any non-valid ignition section
will instead get pruned by the validation.

A bit more background: for oc explain to work, we need to have

preserveUnknownFields: false

which means all fields not defined by the crd schema will be pruned
upon generation of the CR. Since we only define ignition sections
that are allowed, an invalid ignition spec field will simply get
removed when the machineconfig is created. Adding the flag tells
validation that anything under Config is allowed, and we will let
MCC/MCD do the validation instead, like we do today.

Signed-off-by: Yu Qi Zhang <jerzhang@redhat.com>

manifests/machineconfig.crd.yaml

@@ -66,6 +66,7 @@ spec:
             config:
               description: Config is a Ignition Config object.
               type: object
+              x-kubernetes-preserve-unknown-fields: true
               required:
               - ignition
               properties:

pkg/operator/assets/bindata.go

@@ -1324,6 +1324,7 @@ spec:
             config:
               description: Config is a Ignition Config object.
               type: object
+              x-kubernetes-preserve-unknown-fields: true
               required:
               - ignition
               properties: