[SPARK-10642] [PYSPARK] Fix crash when calling rdd.lookup() on tuple keys

JIRA: https://issues.apache.org/jira/browse/SPARK-10642

When calling `rdd.lookup()` on a RDD with tuple keys, `portable_hash` will return a long. That causes `DAGScheduler.submitJob` to throw `java.lang.ClassCastException: java.lang.Long cannot be cast to java.lang.Integer`.

Author: Liang-Chi Hsieh <viirya@appier.com>

Closes #8796 from viirya/fix-pyrdd-lookup.