Behavior changes if initialExecutor is less than minExecutor for dynamic allocation

After SPARK-13723, the behavior changed for spark.dynamicAllocation.initialExecutors less then spark.dynamicAllocation.minExecutors situation.

initialExecutors < minExecutors is an invalid setting,

Before SPARK-13723

If initialExecutors < minExecutors, Spark will throw exception with:

java.lang.IllegalArgumentException: requirement failed: initial executor number xxx must between min executor number xxx and max executor number xxx

This will clearly let user know that current configuration is invalid.

After SPARK-13723

Because we also consider spark.executor.instances, so the initial number is the max value between minExecutors, initialExecutors, numExecutors.

This will silently ignore the situation where initialExecutors < minExecutors.

So at least we should add some warning logs to let user know this is an invalid configuration.

What do you think tgraves, rdblue？