KIJIMR-165. XML config of KeyValueStores does not load defaults.

XML configuration of file-backed KeyValueStores does not properly load Hadoop
default properties.

* XmlKeyValueStoreParser is now parameterized by the current Configuration.
* All file-backed KeyValueStores now implement Configurable.
* XmlKeyValueStoreParser adds default values from current Configuration to conf
  sent to each KeyValueStore.initFromConf() method.