(QENG-968) automate cleanup of zombie ec2 beaker instances...

... generated by jenkins smoketests

- add support to aws_sdk hypervisor to kill zombies that have alive
  longer than a given amount of hours
- make it possible to run beaker without a hosts_file, thus making it a
  script executor
Merge pull request #368 from anodelman/ec2-killer

(QENG-968) automate cleanup of zombie ec2 beaker instances generated by jenkins smoketests