If files-container option is specified:
- All template/env files would be fetched by the heat engine
relative to the files_container and no local files other
than the root template would be sent to server.
- Relative path of environment files would be sent in the
environment_files list.
Also adds the option to template validate.
Change-Id: I1a703ab8798a003365be650886bb78be5af472b7
Story: #1755453
Task: 19319