SPARK-1156: allow user to login into a cluster without slaves

Reported in https://spark-project.atlassian.net/browse/SPARK-1156

The current spark-ec2 script doesn't allow user to login to a cluster without slaves. One of the issues brought by this behaviour is that when all the worker died, the user cannot even login to the cluster for debugging, etc.

Author: CodingCat <zhunansjtu@gmail.com>

Closes #58 from CodingCat/SPARK-1156 and squashes the following commits:

104af07 [CodingCat] output ERROR to stderr
9a71769 [CodingCat] do not allow user to start 0-slave cluster
24a7c79 [CodingCat] allow user to login into a cluster without slaves