Fix issue in ReplSuite with hadoop-provided profile.

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.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #781 from vanzin/repl-test-fix and squashes the following commits:

4f0a3b0 [Marcelo Vanzin] Fix issue in ReplSuite with hadoop-provided profile.