[SPARK-17261] [PYSPARK] Using HiveContext after re-creating SparkContext in Spark 2.0 throws "Java.lang.illegalStateException: Cannot call methods on a stopped sparkContext"

## What changes were proposed in this pull request?

Set SparkSession._instantiatedContext as None so that we can recreate SparkSession again.

## How was this patch tested?

Tested manually using the following command in pyspark shell
```
spark.stop()
spark = SparkSession.builder.enableHiveSupport().getOrCreate()
spark.sql("show databases").show()
```

Author: Jeff Zhang <zjffdu@apache.org>

Closes #14857 from zjffdu/SPARK-17261.

Author: zjffdu

python/pyspark/sql/session.py

@@ -595,6 +595,7 @@ def stop(self):
         """Stop the underlying :class:`SparkContext`.
         """
         self._sc.stop()
+        SparkSession._instantiatedContext = None
 
     @since(2.0)
     def __enter__(self):