set Spark parameter userClassPathFirst  = false #3933 (#3946)
set Spark parameter userClassPathFirst  = false #3933 (#3946)

* changing the default of the spark parameter userClassPathFirst from true -> false
* this parameter is experimental, not recommended, and causing problems for external users
* it was added a long time ago for reasons that are lost to history, and we've been unable to identify any problems that changing it introduces
* fixes https://github.com/broadinstitute/gatk/issues/3933
frank-y-liu/gatk,broadinstitute/hellbender,frank-y-liu/gatk,ksthesis/gatk,frank-y-liu/gatk,frank-y-liu/gatk,magicDGS/gatk,frank-y-liu/gatk,ksthesis/gatk,ksthesis/gatk,frank-y-liu/gatk,magicDGS/gatk,broadinstitute/hellbender,ksthesis/gatk,ksthesis/gatk,broadinstitute/hellbender,magicDGS/gatk,magicDGS/gatk,magicDGS/gatk,magicDGS/gatk,broadinstitute/hellbender,broadinstitute/hellbender,ksthesis/gatk,broadinstitute/hellbender