Use .ci rather than .travis
Update lint config
Add support for multiple ssh_authorized_keys
Add validations and defaults to the $ssh_keys parameter
Use a custom function to ensure we support virtual/exported resources

  * Adds the custom function `resource_sigil()` to return '@' or '@@
      if the currently scoped resource is virtual or exported,
      respectively. Otherwise, returns an empty string

This can be used as follows:

    $sigil = resource_sigil()
    create_resources("${sigil}file", $resource_hash)
Accept arguments in resource_sigil
Remove nested ternary
Attempt to tame function
Reorg a bit
Revert "Use a custom function to ensure we support virtual/exported resources"

This reverts commit 8a5d05ce9aff7ed2b5af6ae205e8c765056f4156.
Remove broken override concept