[SPARK-17336][PYSPARK] Fix appending multiple times to PYTHONPATH from spark-config.sh

## What changes were proposed in this pull request?
During startup of Spark standalone, the script file spark-config.sh appends to the PYTHONPATH and can be sourced many times, causing duplicates in the path.  This change adds a env flag that is set when the PYTHONPATH is appended so it will happen only one time.

## How was this patch tested?
Manually started standalone master/worker and verified PYTHONPATH has no duplicate entries.

Author: Bryan Cutler <cutlerb@gmail.com>

Closes #15028 from BryanCutler/fix-duplicate-pythonpath-SPARK-17336.