Clean hsqldb data directory
- Added gradle task to clean the data directory
  while running "clean" task on spring-xd-dirt
XD-862 Enabled jsonPath Support
XD-875 Shutdown admin-server cleanly
- Re-used the shutdownCleanly() on AdminServer stop()
- Fixed AbstractShellIntegrationTests' adminServer stop
 - Removed explicit adminServer stop() call that hangs
   as the container.stop() would have already stopped
   the admin server while closing its parent context