First implementation of Heat Stacks

This PR implements a simple workflow for heat stacks.

`vagrant up`         => create stacks
`vagrant destroy` => delete stacks

Current format of a stack is 

```ruby
os.stacks = [{
   name: 'stack_name',
   template: '/path/to/my/template.yml'
}]
```

but other attributes should be added later as described in #153 

Once this PR is merged, I will create sub-issues for all heat related work, so that we can split the work and have people contribute.
Actually, heat template can be either yaml or json. But no problem to handle json compatibility in a further pull request.