Add models for cloud orchestration

This PR is split from original PR #333  (to be discarded). It focus only on the models for orchestrations. 
The master PR #523 covers the design and progress of the support for orchestration including AWS CloudFormation and OpenStack Heat.
![orchestration](https://cloud.githubusercontent.com/assets/7047579/4719534/6144c4e0-592a-11e4-9dc1-7d882f00f947.png)

Prefer the arel approach...In fact, this method can just be a scope instead of a `find_` method.

```ruby
def in_use
  joins(:stacks).uniq
end
```

I think that's the code.  @tenderlove @jrafanie Can you double check my work?