reject offers if there's no etcd servers to launch

[jdef]

in some environments, hoarding offers for up to 5s is problematic.

the 5s in this case is based on chillFactor: https://github.com/mesosphere/etcd-mesos/blob/4c7b93511d8ea6fdefe714f6f1ac826cb51ec2f8/scheduler/scheduler.go#L430