Fix invalid config JSON and enforce canonical config format

config/disk-log-message-filelog.json contained trailing commas and no
trailing newline, so it could not be parsed by encoding/json and NPD
would fail to start when configured with it.

- Fix the invalid JSON
- Add TestShippedConfigs, which verifies that every shipped JSON config
  under config/ (top level and one directory down) parses (catching
  trailing commas, UTF-8 BOMs and other syntax errors) and is in the
  canonical format (2-space indent, trailing newline), showing a diff on
  failure
- Add `make fmt-configs` to rewrite nonconforming files in place
  (UPDATE_EXPECTED=y)
- Normalize the existing config files to the canonical format
  (whitespace-only changes)

Makefile

@@ -15,7 +15,7 @@
 # Build the node-problem-detector image.
 
 .PHONY: all \
-        lint vet fmt version test e2e-test \
+        lint vet fmt fmt-configs version test e2e-test \
         build-binaries build-container build-tar build \
         docker-builder build-in-docker \
         push-container push-tar push release clean depup \
@@ -144,6 +144,9 @@ fmt: $(GOLANGCI_LINT)
 	find . -type f -name "*.go" | grep -v "./vendor/*" | xargs gofmt -s -w -l
 	$(GOLANGCI_LINT) run --config .golangci.yml --fix ./...
 
+fmt-configs:
+	UPDATE_EXPECTED=y go test -run TestShippedConfigs ./pkg/systemlogmonitor
+
 version:
 	@echo $(VERSION)