Check that 'heat_template_version' is a HOT version

Previously, any value could be specified as the 'heat_template_version'.
Check that 'heat_template_version' is a valid HOT version and raise a
ValueError if it is not.

Change-Id: I7f9a7acd52cf6273a4730091c08b8cd1a5e07961
Closes-Bug: #1277215
Merge "Check that 'heat_template_version' is a HOT version"