Add support for snippets

[vaijab]

There is a lot of repetition between stack templates. It would be great to have a way of including code snippets from files which could be reused in different stacks.

For instance having snippets/udpate-ca-certificates.service snippet which could be reused in multiple templates in user-data section.

[lewismarshall]

👍 for this.

Another related but possibly separate use case - including data generated firewall rules... maybe this should be a from data function...?