cluster: don't return the ID of the node in CreateContainer

It should match the return from CreateDocker in go-dockerclient.
cluster: add Scheduler type

Also including a round robin implementation, that will be used as the
default scheduler.

Related to #8.
cluster: fix go vet warning
scheduler: add method Nodes to the Scheduler interface

Related to #8.
cluster/scheduler: gofmt -s -w .
cluster: use Scheduler

Closes #8.