merge.py --hot to process and generate HOT

This adds a --hot flag to merge.py which will assume source templates
are in the HOT format, and generate a HOT template artifact.

Tests have also been ported to HOT, along with some minor corrections
to the existing tests to make the source and result templates valid
heat templates.

Partial-Blueprint: tripleo-juno-remove-mergepy

Change-Id: If18ff79f89456123c884a1ab2f910ce4cc9a6e0b