Allow disabling hcl2 interpolation for string literals

[notnoop]

HCLv2 added more comprehensive support for expression interpolation within strings. Users can insert variable references (e.g. ${var.user_field}) or expressions in here-doc style syntax:

template = <<EOT
%{ for ip in var.ips }
server ${ip}
%{ endfor }
EOT

This interpolation though comes with downsides. Nomad templates frequently embed shell/perl scripts or embedded nijna-like templates that can conflict with the use of ${ or %{. Users must escape references of ${ and %{, a tedious task, specially for users upgrading from pre-1.0.0 nomad versions.

We should consider adding a mechanism to disable HCLv2 interpolation processing; e.g. adopt the bash heredoc mechanism to disable variables, by using ', e.g. <<'EOF'.

The change needs to be coordinated with the hcl library.

This issue was raised in #10743 and #9838 (comment) .

[idrennanvmware]

@notnoop - just wanted to follow up if there had been any further movement on this? We're still stuck on HCL1 jobs and getting deeper and deeper in to having a hard time switching to HCL2 without this kind of feature

Thanks!

[tgross]

I'm going to close this issue as a duplicate of #9838. I've left some notes there recently on the challenge involved in implementing this, which will likely require upstream changes to HCL syntax.