Add Plan versioning management

* Version can now be passed in attribute of the Plan YAML File.
Version attribute is a mandatory field.
* According to the version specified, the appropriate WSME Plan
and Plan Handler are created and called.
* Plan validation is done by the WSME framework.
* How to add a new Plan version (e.g with version 2) ?
 - Add your Plan_v2 WSME object in datamodels
 - Add your Plan_v2_handler in handlers
 - Add init_plan_v2 method in Plan controller.

Change-Id: I7fcc6e11391b9f026768a4afafaaa638c43ec303
Closes-Bug: #1291114
Merge "Add Plan versioning management"