Refactor Heat Templates, Definitions and Entrypoints

Currently, Magnum uses Heat templates to instantiate container clusters. The glue between Magnum and selecting the appropriate Heat template is a combination of a template definition and an entrypoint. Additional details on this concept can be found here:

https://github.com/openstack/magnum/blob/master/contrib/templates/example/README.rst

A considerable amount of code duplication exists among templates of the same coe type. To reduce this duplication and make the Heat template approach more scalable, flexible, etc. this blueprint suggests redesigning Magnum's approach to using Heat templates. As part of this effort, the Heat templates should be redesigned to be more compossible. For example, instead of having 6 Kubernetes Heat templates, 2 for each supported OS, a pair of Heat templates should be used for all supported OS's within the Kubernetes coe type. In this scenario, the particular OS type should be passed to the top-level Heat template and Heat use the correct fragment to orchestrate OS-specific parameters.