Bug 1450965

Cause: In a legacy (non-convergence_engine) stack update, resource definitions are copied into the existing template one at a time as resources are updated. The full template is copied at the end of the update. If the new template is of a newer version than the old and contains resources that use features (e.g. intrinsic functions) not available in the old version, then those resources definitions are unreadable in the context of the old template. Consequence: When updating a legacy stack using a newer version of the template format, if the update failed at some point after updating/creating one or more resources relying on features not included in the old template version, then Heat would be unable to load and further update the stack. Fix: When the old an new templates have differing versions, we now use the lexicographically highest version for both. Result: After an update failure, the partially updated template will generally be interpreted using the latter of the two template versions. So 'ocata' instead of 'newton'; '2017-02-24' instead of '2016-10-14'; 'ocata' instead of (the equivalent) '2017-02-24'. The exception is that e.g. the older 'newton' will be preferred instead of the newer '2017-02-24' - for this reason it is recommended not to switch from using release names to dates in template versions. Switching from dates to release names works correctly when moving to later versions.