Make add_member use **kwargs and doc string update

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 add_member use **kwargs.
Also update doc string for the same.

Partially implements blueprint consistent-service-method-names

Change-Id: I67810b4e9c313611393e896f68d3b4a1dda77bea
1 file changed