CDAP-395: StateStore - Enable Programs to store/retrieve states during runtime.

* Create PreferencesTable to store Preferences (currently used for StateStore)
* Create ConfigService (SystemService) that will have a HTTP Service with endpoints for Preferences API. Since StateStore does not expose REST endpoints, we currently have only a PingHandler attached to it. More handlers will be added during the course of Preferences API work.
* Unit Test to verify StateStore feature.

JIRA: https://issues.cask.co/browse/CDAP-395
Bamboo: https://builds.cask.co/browse/CDAP-DUT198
I know this is not a part of your PR but there is no documentation for this test class. 
Can you please include it when you are at it.