Merge branch 'stable-2.15' into stable-2.16

* stable-2.15:
  ReadOnlyByHttpIT: Extend to cover both 'readonly' and 'readonly~readonly'
  ReadOnlyByHttpIT: Extend assertions on response content
  ReadOnly: Format with google-java-format
  Allow endpoint "config/server/readonly"
  REST: Report state from PUT and DELETE

Adjust ReadOnlyByHttpIT to the changed package of ConfigSuite.

Change-Id: I34c0d7fc093e79dd631ad5090dda136cbf44b533