Add "network rbac set" command which just supports
setting a target project.
Also, This patch adds the doc, unit test and functional test.
But there is a bug of showing network RBAC
https://bugs.launchpad.net/python-openstacksdk/+bug/1608903
We need to skip the functional test before this bug fixed.
Change-Id: I756f448bb333cf1098a735e57a1c5dc4edf195d4
Partially-Implements: blueprint neutron-client-rbac