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)

also can be Arelized:

```ruby
joins(:stacks).where(OrchestrastionStack.arel_table[:template_id] => nil))  # or is it .eq(nil)
```

I'm not sure if joins is the right thing here or `references`.  Can't remember which version of AR has that. @tendelove @jrafanie Help?