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)

config/exporter/stackdriver-exporter.json

@@ -1,8 +1,8 @@
 {
-	"apiEndpoint": "monitoring.googleapis.com:443",
-	"exportPeriod": "60s",
-	"metadataFetchTimeout": "600s",
-	"metadataFetchInterval": "10s",
-	"panicOnMetadataFetchFailure": false,
-	"customMetricPrefix": ""
+  "apiEndpoint": "monitoring.googleapis.com:443",
+  "exportPeriod": "60s",
+  "metadataFetchTimeout": "600s",
+  "metadataFetchInterval": "10s",
+  "panicOnMetadataFetchFailure": false,
+  "customMetricPrefix": ""
 }