Adds included-template-dir param for resolving template FileIncludes

In I587fa7a826f93f89e8e5c266af7f5765438fe738 there is a problem with
the overcloud.yaml Make target. Once the merge.py script is moved from
where the templates are, FileInclude paths cannot be resolved relative
to the script. The included-template-path defaults to './'

Change-Id: I220c5e52d8062e98aa28a1c582e29c7e844bc9ae