[SPARK-8852] [FLUME] Trim dependencies in flume assembly.

Also, add support for the *-provided profiles. This avoids repackaging
things that are already in the Spark assembly, or, in the case of the
*-provided profiles, are provided by the distribution.

The flume-ng-auth dependency was also excluded since it's not really
used by Spark.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #7247 from vanzin/SPARK-8852 and squashes the following commits:

298a7d5 [Marcelo Vanzin] Feedback.
c962082 [Marcelo Vanzin] [SPARK-8852] [flume] Trim dependencies in flume assembly.