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)

CFN template can be used by either Amazon or OpenStack. How to deploy is only provider specific but not template type specific. It is hard here to do subclass either by provider nor by template type.