Make v2,v3 create_role use **kwargs & doc string

As we discussed on
http://lists.openstack.org/pipermail/openstack-dev/2015-July/068864.html
All http POST/PUT methods need to contain **kwargs as their arguments.

This patch makes create_role of v2,v3 identity_client use **kwargs.
Also update doc string for the same.

Partially implements blueprint consistent-service-method-names

Change-Id: I876e9e5b8aea37076b180a7857dd79bb6d7c1f10
11 files changed