[SPARK-2871] [PySpark] add RDD.lookup(key)

RDD.lookup(key)

        Return the list of values in the RDD for key `key`. This operation
        is done efficiently if the RDD has a known partitioner by only
        searching the partition that the key maps to.

        >>> l = range(1000)
        >>> rdd = sc.parallelize(zip(l, l), 10)
        >>> rdd.lookup(42)  # slow
        [42]
        >>> sorted = rdd.sortByKey()
        >>> sorted.lookup(42)  # fast
        [42]

It also clean up the code in RDD.py, and fix several bugs (related to preservesPartitioning).
Minor nit, but I'd drop the 'of' and just say "Cannot reduce() empty RDD"