add exclusion for old servlet-api on hadoop-client in core

Maven tests are failing in jenkins using the following profile and label:
-Dhadoop.version=2.0.0-mr1-cdh4.1.2 -Dlabel=centos

This is due to the cdh4 hadoop-client dep leaking in a transitive dependency on javax.servlet:servlet-api:2.5. This results in the repeated exceptions in the test run of:

Exception encountered when invoking run on a nested suite - class "javax.servlet.FilterRegistration"'s signer information does not match signer information of other classes in the same package *** ABORTED ***

https://github.com/apache/spark/pull/1271