allow custom private terraform snippets during the use of terraform e…

[jschaul]

…nvironment usage

[arianvp]

I think this demonstrates the smell I was a bit worried about and we broadly discussed a few months ago.

@jmatsushita this mostly seems to be a work around for the pattern that we only have tfvars in cailleach at the moment. I think my suggestion to have only modules in wire-server-deploy and have actual terraform code in cailleach call those modules; instead of using just .tfvars is probably a better way to tackle this. Thoughts? That way people can always add additional terraform code in cailleach if needed.

I'm ok with merging this for now; but I think the better way is to actually have the main terraform file in cailleach and have that call modules in wire-server-deploy so that we don't need these kind of workarounds but just let the tool do its job.