AWS parameter type name instead of CFNType

Right now, if you want to specify a variable as a CloudFormation parameter, you would import the appropriate CFNType, then use that as the type field. For example:

```python
from stacker.blueprints.variables.types import EC2VPCId

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

This works fine, but I think we could make a little simpler and easier to use. If instead, you could do the following:

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

And stacker would internally convert it to a `EC2VPCId`. This behavior would be pretty similar to what we had in 0.8.