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

...ace-id as a PathParam. Added validations for namespace-id.


Summary:
https://issues.cask.co/browse/CDAP-900
https://issues.cask.co/browse/CDAP-1076

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 it changes the ``name`` field in ``NamespaceMetadata`` to ``id``.
This is a BAD_REQUEST and not a conflict. The idea is that we don't want users to make this request again. Conflict would imply that there is a problem with the internal state and there is a possibility of the request to go through at a later stage. But the request would never succeed.