Heat templates need to be versioned

Heat templates are constantly changing in Magnum, either due to refactoring work or adding new features.

Everytime we change a heat template, it triggers a full heat stack redeployment when a user updates a bay which uses the heat template.

We need to version heat templates and associate each bay with the version used at the time of bay creation. This allows users to continue using a bay and scale it without heat rebuilding the entire stack when we make updates to a heat template.