Split EC2 ID validator to validator per resource type.

Now there is only one method to validate EC2 id: validate_ec2_id().
It's used to validate both the Instance ID as well as Volume ID for
valid EC2 ID format. However exception class raised in both cases
is "InvalidInstanceIDMalformed" and EC2 error code is
InvalidInstanceID.Malformed.

This patch addresses this problem by adding two new methods:
* validate_instance_id()
* validate_volume_id()
These methods use validate_ec2_id() and raise appropriate
exception as well as appropriate EC2 error code based on
AWS EC2 Error Code documentation:
http://docs.aws.amazon.com/AWSEC2/latest/APIReference/api-error-codes.html

Change-Id: I55591b8b643bb316e5001e645d71c13094106e96
Closes-Bug: #1199308
Merge "Split EC2 ID validator to validator per resource type."