Add support for custom AMI's if a non-default is needed
Merge pull request #21 from chrisjs/chronos-multinode-zk

Use zk hosts for mesos master rather than hardcoded localhost in multinode Vagrantfile
use vagrant cloud boxes. #19
Merge pull request #22 from chrisjs/configurable_ami

Add support for custom AMI's if a non-default is needed
add base box info in README.
Merge branch 'pre-baked-images'

Conflicts:
	multinodes/Vagrantfile
	standalone/Vagrantfile
put comments for custom_ami in Vagrantfiles.

It seems that vagrant-aws plugin can't override `aws.ami` when we set vagrant box which supports was provider.