[CDAP-900] [CDAP-1076] Changed the namespace create API to accept namesp...

...ace-id as a PathParam. Added validations for namespace-id.


Summary:
1. Changed the namespace create API to ``PUT /v3/namespaces/{namespace-id}``
2. Added validations for namespace-id - No special characters other than '-' and '_', no "default" or "system" literals.

Impacts a large number of files primarily because we refer to the key for namespaces as ``id`` instead of ``name``.