Goal: 

• To support using the latest snapshots for templates, the update_template_content task should be modified to optionally fetch the latest snapshots for each repository, rather than fetch by the date.
• Check use_latest parameter on template before fetching by date

Acceptance Criteria:

• When creating or updating a template that is set to use the latest snapshot, the distributions created should serve the latest snapshot for every repository in the template
• Fetching snapshots for a template via the api, i should get back the latest if use_latest is set