[SPARK-18687][PYSPARK][SQL] Backward compatibility - creating a Dataframe on a...

[SPARK-18687][PYSPARK][SQL] Backward compatibility - creating a Dataframe on a new SQLContext object fails with a Derby error

Change is for SQLContext to reuse the active SparkSession during construction if the sparkContext supplied is the same as the currently active SparkContext. Without this change, a new SparkSession is instantiated that results in a Derby error when attempting to create a dataframe using a new SQLContext object even though the SparkContext supplied to the new SQLContext is same as the currently active one. Refer https://issues.apache.org/jira/browse/SPARK-18687 for details on the error and a repro.

Existing unit tests and a new unit test added to pyspark-sql:

/python/run-tests --python-executables=python --modules=pyspark-sql

Please review http://spark.apache.org/contributing.html

 before opening a pull request.

Author: Vinayak <vijoshi5@in.ibm.com>
Author: Vinayak Joshi <vijoshi@users.noreply.github.com>

Closes #16119 from vijoshi/SPARK-18687_master.

(cherry picked from commit 285a7798)
Signed-off-by: Wenchen Fan <wenchen@databricks.com>