[SPARK-11686][CORE] Issue WARN when dynamic allocation is disabled due to...

[SPARK-11686][CORE] Issue WARN when dynamic allocation is disabled due to spark.dynamicAllocation.enabled and spark.executor.instances both set

Changed the log type to a 'warning' instead of 'info' as required.

Author: Ashwin Swaroop <Ashwin Swaroop>

Closes #9926 from ashwinswaroop/master.