Receive namespace and codegen_model flags from the cli

[chuchocd85]

Issue #, if available:

Description of changes:
Add ability to consume the namespace and codegen model from the cloudformation-cli.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[johnttompkins]

Travis looks to be failing here on the _prompt_for_namespace method any idea why?

[chuchocd85]

Travis looks to be failing here on the _prompt_for_namespace method any idea why?

Looking at the runs in travis the problem is when importing from rpdk.core.init. I don't know exactly how importing from another repo works but my guess is that Travis is pulling master which does not have the methods (yet) being imported.