[SPARK-1805] [EC2] Validate instance types

Addresses [SPARK-1805](https://issues.apache.org/jira/browse/SPARK-1805), though doesn't resolve it completely.

Error out quickly if the user asks for the master and slaves to have different AMI virtualization types, since we don't currently support that.

In addition to that, we print warnings if the inputted instance types are not recognized, though I would prefer if we errored out. Elsewhere in the script it seems [we allow unrecognized instance types](https://github.com/apache/spark/blob/5de14cc2763a8211f77eeb55940dec025822eb78/ec2/spark_ec2.py#L331), though I think we should remove that.

It's messy, but it should serve us until we enhance spark-ec2 to support clusters with mixed virtualization types.

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #4455 from nchammas/ec2-master-slave-different-virtualization and squashes the following commits:

ce28609 [Nicholas Chammas] fix style
b0adba0 [Nicholas Chammas] validate input instance types