Simpler shorthand for Value=foo parameters

This is a work in progress PR that does two things.


1) For any structure of a single scalar with a key name of "Value" you can just provide the value directly (`--instance-type m1.small` instead of `--instance-type Value=m1.small`)
2) Allow boolean parameters to be provided without a value (`--ebs-optimized/--no-ebs-optimized` instead of `--ebs-optimized Value=true`).

This fixes item 1 of https://github.com/aws/aws-cli/issues/484 but not item 2.  Item 2 will likely be another pull request.


The only thing left is to integrate the doc generation to only document the simpler syntax.