(MAINT) nits in beaker ec2 support

- correctly identify zombies that are over X hours old
- better notification of what instances are being examinged for possible
  termination
- properly notify user of missing aws_access_key_id and
  aws_secret_access_key during initialization
Merge pull request #408 from anodelman/ec2

(MAINT) nits in beaker ec2 support