Configurable unit templates

[bmorton]

At the present time, we only support the one hard-coded unit template. This will probably work for lots of stateless service use-cases, but it definitely won't cover everything. There are a few different ways we could handle this:

• Have more templates come out of the box and allow contributions for new, universal templates
• Allow users to provide templates at application runtime
• Allow the parts that need to be customized to be part of the POST payload
• Allow a URL to a unit file to be passed in the POST payload