Add @validation.require_openstack() validator

This patch removes from @base.scenario(admin_only=False) argument.
That actually doesn't work and doesn't cover situation when we need
only users or admins or admins & users..

Add proper @require_openstack validator that actually checks
everything that is required.
So you can specify that only users or admin is required or both
users and admin are required.

Add missing "users" context in rally-scenario for scenarios that
use it.

Add missing "sla" sections in benchmarks

Reduce amount of iterations in "rps" benchmark

Change-Id: Ie92dbd473ee94c2073401c6da8dc2771c9dfaed8