Implement a basic scheduler for Higgins

Implement a very simple scheduler for Higgins, which schedule a container to a host. The initial implementation should be simple. For example, choose a host in a round-robin way. It is adviced to make the interface the same as nova-scheduler, so that we can migrate the nova-scheduler earlier later.

NOTE: This BP moved to https://blueprints.launchpad.net/zun/+spec/basic-container-scheduler