cfn init should enforce namespace restrictions

If I attempt to create a Resource with a [reserved name](https://docs.aws.amazon.com/cloudformation-cli/latest/userguide/resource-type-schema.html#resource-type-schema-properties), I should be warned or the CLI should refuse the input.

````bash
Initializing new project
What's the name of your resource type?
(Organization::Service::Resource)
>> AWS::Cloud9::Membership
Select a language for code generation:
[1] python36
[2] python37
(enter an integer):
>> 2
Use docker for platform-independent packaging (Y/n)?
This is highly recommended unless you are experienced
with cross-platform Python packaging.
>> Y
Initialized a new project in /home/ec2-user/aws-cloud9-membership
````