[SPARK-5636] Ramp up faster in dynamic allocation

A recent patch #4051 made the initial number default to 0. With this change, any Spark application using dynamic allocation's default settings will ramp up very slowly. Since we never request more executors than needed to saturate the pending tasks, it is safe to ramp up quickly. The current default of 60 may be too slow.

Author: Andrew Or <andrew@databricks.com>

Closes #4409 from andrewor14/dynamic-allocation-interval and squashes the following commits:

d3cc485 [Andrew Or] Lower request interval