PySpark PowerIterationClustering test failure due to bad demo data.
If the data is small,  PowerIterationClustering will behavior indeterministic.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #7177 from yanboliang/spark-8765 and squashes the following commits:

392ae54 [Yanbo Liang] fix model.assignments output
5ec3f1e [Yanbo Liang] fix PySpark PowerIterationClustering test issue