Use restricted access context client for setup operations

Introduce 'user_cloud' as a set of clients restricted to user-level
access. Keep 'cloud' as a global admin client for operations that
require escalated privileges (like access to hypervisors extension).
Add floating ips by ServerManager method

Instead of adding floating ip by Server.add_floating_ip(), use
ServerManager.add_floating_ip() method for consistency and more
accurate context control.
Fixed restriction of right on resources management
Don't create networks per tenant for FlatDHCP network manager