... initial number

Author: Sandy Ryza <sandy@cloudera.com>

Closes #4051 from sryza/sandy-spark-4585 and squashes the following commits:

d1dd039 [Sandy Ryza] Add spark.dynamicAllocation.initialNumExecutors and make min and max not required
b7c59dc [Sandy Ryza] SPARK-4585. Spark dynamic executor allocation should use minExecutors as initial number