rancher2-deployment: allow splitting up of the deployment template

[jzaefferer]

Our ops/deploy/deployment.tmpl.yml is getting quite big. Splitting that up into multiple files, by service, could help.

Maybe like this:

- ingress.tmpl.yml # kind: Certificate and kind: Ingress
- frontend.tmpl.yml
- auth.tmpl.yml
- [service-n].tmpl.yml

Deploying with kubectl -f dir/ could be an option, but that could break template logic and variables that all files need.

Maybe concatenate multiple files into one, before deploying it. Like 001-service-a.tmpl.yml, 002-service-b.tmpl.yml, then cat *.yml > deployment.tmpl.yml.