Unable to specify 'load_tag' dynamically from the pipeline task

[holgerstolzenberg]

Currently it is not possible to gather the load_tag dynamically during the pipeline execution by for example reading it from a previously generated file (like done with tag_file).

[vito]

This should be done by using the load_var step:

plan:
- # ...
- load_var: foo
  file: path/to/tag
- put: resource
  params: {load_tag: ((.:foo))}

The idea behind the load_var step is to allow resource type implementations to simplify and focus on static values, rather than having to implement foo and foo_file forms of every param.

This step was newly added in v6.0 - it's currently marked experimental, but its RFC has since been merged, so I think we can un-flag it. 👍