Proper rendering of credentials

[claesgill]

Feature description

We need proper rendering of credentials because we want to avoid storing credentails in clean text in terraform state.

Why is it needed?

We don't want to show credentials in clean text.

Suggestion(s)/solution(s) [Optional]

Partly done in the Presto module: https://github.com/fredrikhgrelland/terraform-nomad-presto/blob/master/conf/nomad/presto.hcl#L327

Definition of done

• Rendering credentials in the nomad job (.hcl file)

Checklist (after created issue)

• Added label(s)
• Added to project
• Added to milestone