Make identity v2 user_client use **kwargs

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 identity v2 user_client use **kwargs.

Also adding some doc link.

Partially implements blueprint consistent-service-method-names

Change-Id: I058e74a205ff9223113ad9fed8be61e6b66e7045
8 files changed