Fix two issues in ReplSuite.

First, local cluster mode was failing because the backend impl was
not passing correct arguments to start the executors (it was missing
the number of cores).

Second, when building the assembly with the maven "hadoop-provided"
profile, the executors were failing to come up because Hadoop classes
were not found in the classpath anymore; so add them explicitly to
the classpath using spark.executor.extraClassPath. This is only
needed for the local-cluster mode, but doesn't affect other tests,
so it's added for all of them to keep the code simpler.