--ebs-volume-type and --provisioned-iops support

Adds support for --ebs-volume-type and --provisioned-iops parameters
midnight oil

let it buuuuuurn (this fix works)
add validations for provisioned iops
Added validation for ebs volume type and provisioned-iops option
Added rspec tests for ebs volume type and provisioned-iops option
Added support for gp2 ebs volume type
Added rspec tests for gp2 ebs volume type
Merge pull request #219 from ClogenyTechnologies/kd/Provisioned-IOPS

Add provisioned IOPS to knife-ec2