Prevent users from using "{}" in template names

I was experimenting with unit file templating and built a small tool to generate templates and then start services using {1,2,3} syntax. But accidentally somehow my shell escaped the {} making fleetctl to submit an actual unit named @{1,2,3}.service.

This made me have to go through all sorts of hidden keys and places to clean it up. In some places it actually  fleet took it as 1,2 and 3 but in some places not. Maybe there should be some checks for this?
This is intentionally not exhaustive; the correct place for that is the unit tests covering these functions. This is just a simple functional test of the common cases we've seen to ensure that fleetctl does the validation.