[SPARK-10086][MLLIB][STREAMING][PYSPARK] ignore StreamingKMeans test in PySpark for now

I saw several failures from recent PR builds, e.g., https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/50015/consoleFull. This PR marks the test as ignored and we will fix the flakyness in SPARK-10086.

gliptak Do you know why the test failure didn't show up in the Jenkins "Test Result"?

cc: jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #10909 from mengxr/SPARK-10086.