Make PySpark ```FPGrowthModel.freqItemsets``` consistent with Java/Scala API like ```MatrixFactorizationModel.userFeatures```
It return a RDD with each tuple is composed of an array and a long value.
I think it's difficult to implement namedtuples to wrap the output because items of freqItemsets can be any type with arbitrary length which is tedious to impelement corresponding SerDe function.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #5614 from yanboliang/spark-6827 and squashes the following commits:

da8c404 [Yanbo Liang] use namedtuple
5532e78 [Yanbo Liang] Wrap FPGrowthModel.freqItemsets and make it consistent with Java API