Refactor the way the prov package access Tosca informations

[loicalbertin]

As: Yorc dev team
I want to: refactor the way the prov package access Tosca informations
So that: It will be more maintainable

AC1: Elaborate a current state of the usage of the deployments package within executors
AC2: The idea is to provide to executors a structure that is totally serializable and do not need to access the deployment package anymore.
AC3: Every things (like operation inputs) should be pre-resolved
AC4: executors should have access to a reasonably limited set of call back operations to update attributes / states on

Note for the record: Current implementation prevent us to easily implement new storage backends in plugins