Make target to validate a template

Make target to validate a template

This adds make targets to run heat template-validate against the
generated templates.

These targets assume that credentials to a valid heat endpoint
have been sourced.

The target validate-all will validate every template in
generated_templates, but there is also a target created for each
template, for example:

  make validate-overcloud.yaml

This is a useful aid for
Partial-Blueprint: tripleo-juno-remove-mergepy

Change-Id: Id101402b8b2fa0bc6058c5bc4b7474f0f6a158ad