Only fetch required ips per switch describe instead of all

[majst01]

During switch describe, which is called from every metal-core every 20 sec, all ips are listed and later filtered by project.
This is a lot of useless data fetched.

This was also found during the dualstack implementation, test was not adopted yet.

TODO:

• write integration test with real rethinkdb in place which proves correctness

[Gerrit91]

Suggested change

	ips := make([]metal.IP, 0)
	ips := metal.IPs{}