[SPARK-6246] [EC2] fixed support for more than 100 nodes

This is a small fix. But it is important for amazon users because as the ticket states, "spark-ec2 can't handle clusters with > 100 nodes" now.

Author: alyaxey <oleksii.sliusarenko@grammarly.com>

Closes #6267 from alyaxey/ec2_100_nodes_fix and squashes the following commits:

1e0d747 [alyaxey] [SPARK-6246] fixed support for more than 100 nodes