Add support for specifying variable types using named parameter types

[ejholmes]

Closes https://github.com/remind101/stacker/issues/546

This just makes it a little easier to use CFNType's, by just specifying the name of the type you want for the parameter. The following now works:

VARIABLES = {
    "VpcId": {
        "type": "AWS::EC2::VPC::Id"
    }   
}

And it'll be automatically normalized to:

VARIABLES = {
    "VpcId": {
        "type": EC2VPCId
    }
}