Cleanup network schedulers

This does the cleanup described in https://github.com/axsh/wakame-vdc/issues/138

I deleted these schedulers:

* FlatSingle
* NatOneToOne
* VifTemplate

They were made before we had the vifs parameter. They were currently broken and the VifsRequestParam scheduler pretty much replaces them all. I don't think it's worth maintaining them at this point so I deleted them.

I removed the methods Instance#add_nic and NetworkVif#attach_to_network from the schedulers. These methods forced all schedulers to either induce a circular dependency or follow the same hard coded ip/mac address assignment algorithm. This change allows those assignments to be done by schedulers without the circular dependency.