ReadOnlyConfiguration throws exception if any Configuration in current context calls addDefautlResource

Reported by psomogyi up on HBASE-19295, the issue that added CpEnv returning a read-only Configuration.

Issue is that hadoop Configuration registers every created Configuration in a static REGISTRY on construction. A call to addDefaultResource – e.g. sqoop does this in tests to get its configurations into the mix – results in a call to every registered Configurations loadResource.. which is disallowed in our read-only Configuration.