MAHOUT-1529: spark-shell - cleanup sdc

Renaming @sc to @sdc has left open the shutdown part. Spark
depends on 'implicit sc' to be set by the shell invoker.

  mahout> Stopping spark context.
  <console>:29: error: not found: value sc
                sc.stop()
                ^

So override sparkCleanUp() and call @sdc.stop()

Signed-off-by: Anand Avati <avati@redhat.com>