Add methods to attach and detach routes

Add attach_route, attach_routes, detach_route, and detach_routes methods
to the models.

In the case of models/f5.rb, factor attach_routes and detach_routes
methods out of the create_routes and delete_routes methods.

In the case of models/lbaas.rb, use a new LBaaS call for attach_routes.
(There is currently no interface with which one can implement
detach_routes.)

Make the controllers explicitly invoke these methods to attach and
detach routes after creating and before deleting them, respectively.

Add a new VIRTUAL_SERVER configuration setting to load-balancer.conf,
which the controllers read and which determines the virtual server to
which new routes will be attached.