Either get rid of or stabilize HCL repo fork

[johakoch]

Couper uses the https://github.com/avenga/hcl fork of https://github.com/hashicorp/hcl.
Either

• try to incorporate Allow optional labels avenga/hcl#2 into https://github.com/hashicorp/hcl, or
• stabilize the fork by e.g. regularly updating it and creating a test setup.

[johakoch]

I opened an issue: hashicorp/hcl#666

[johakoch]

The fix we currently have (a third token "optional" following the second, kind token "label") is not conformant with gohcl/doc.go:

// Within each tag there are two comma-separated tokens. The first is the
// name of the corresponding construct in configuration, while the second
// is a keyword giving the kind of construct expected.

We could also use another kind, like "optional_label", to get the same effect. So even if the extraction of name and kind (in https://github.com/hashicorp/hcl/blob/57f8bbf184a628a6dedb36520db1c90bfab7fb06/gohcl/schema.go#L144 and following) becomes stricter, it should better not fail.