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.
```
undefined method `each' for nil:NilClass (NoMethodError)
```
When there's no stack declared in the Vagrantfile. Should be fixed by
```ruby
config.stacks.each do |stack|
  ...
end unless config.stack.nil?
```