ACCUMULO-3012 Improve DefaultConfiguration

* The map of default properties created during the iterator() method each time is now
  static and immutable.
* A unit test is added.
* Accompanying changes were made to ConfigSanityCheck:
  - The validate method takes in an Iterable of entries instead of a Configuration.
  - Validation failure throws a new SanityCheckException class instead of RuntimeException.
  - A unit test is added.