Fix security group api

Added the following apis.

* put 'network_vifs/:vif_id/add_security_group'
* put 'network_vifs/:vif_id/remove_security_group'

Updated the load balancer api to use the above when registering instances.

Deprecated the security groups part in "put 'instances/:inst_id'" and introduced a service type check. This is because the LB security groups got deleted if the instance was registered to a load balancer.

In the future we should rewrite the edit_instance dialog in the GUI so it uses the new security group api in network_vifs instead.

This does *NOT* change LB security group behaviour. Load balancers just use the new api for the same behaviour as before.