SPARK-2332 [build] add exclusion for old servlet-api on hadoop-client in core

Fix for class of test suite failures in jenkins

Author: Peter MacKinnon <pmackinn@redhat.com>

Closes #1271 from pdmack/master and squashes the following commits:

cfe59fd [Peter MacKinnon] exclude servlet-api in hadoop-client for sbt
6f39fec [Peter MacKinnon] add exclusion for old servlet-api on hadoop-client in core