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.
Stacks must be deleted after instances. Be careful, in delete action, execution order is as it is written in the code (unlike create action).