Implement stack dependencies

[vaijab]

It would be great to be able to define dependencies between stacks. For instance, coreos stack depends on infra stack.

Stack dependencies could be defined in a template itself, for example templates/coreos.yaml:

---
depends:
  - infra
  - frontend-elb

---

AWSTemplateFormatVersion: '2010-09-09'                         
Description: CoreOS stack
Resources:                                                     
# here goes the rest of the template