SPARK-1106: check key name and identity file before launch a cluster

I launched an EC2 cluster without providing a key name and an identity file. The error showed up after two minutes. It would be good to check those options before launch, given the fact that EC2 billing rounds up to hours.

JIRA: https://spark-project.atlassian.net/browse/SPARK-1106

Author: Xiangrui Meng <meng@databricks.com>

Closes #617 from mengxr/ec2 and squashes the following commits:

2dfb316 [Xiangrui Meng] check key name and identity file before launch a cluster