and by extension, the ami-list

Useful for using alternate spark-ec2 repos or branches.

Author: Florian Verhein <florian.verhein@gmail.com>

Closes #4385 from florianverhein/master and squashes the following commits:

7e2b4be [Florian Verhein] [SPARK-5611] [EC2] typo
8b653dc [Florian Verhein] [SPARK-5611] [EC2] Enforce only supporting spark-ec2 forks from github, log improvement
bc4b0ed [Florian Verhein] [SPARK-5611] allow spark-ec2 repos with different names
8b5c551 [Florian Verhein] improve option naming, fix logging, fix lint failing, add guard to enforce spark-ec2
7724308 [Florian Verhein] [SPARK-5611] [EC2] fixes
b42b68c [Florian Verhein] [SPARK-5611] [EC2] Allow spark-ec2 repo and branch to be set on CLI of spark_ec2.py