[data/values] support specifying data values via cmd args that are not present in file data values

[cppforlife]

in some cases it's useful to specify data values that are not present in file data values via cmd arg. since cmd args are overlayed on top of file data values they need to have annotation overlay/match missing_ok=True set on them.

ytt -f config.yml -v something+.namespace=blah

we can try to assign special character (+) that could be used to indicate that.

TBD:

• should this also apply to env vars? (env vars are constrained to a-zA-Z0-9_ unfortunately)

👍 "I have this use-case and would be served by having this feature implemented"
👎 "I think implementing this feature would do more harm than good."

[cppforlife]

+ was chosen to be friendly to shells.

[pivotaljohn]

User (specifically a Configuration Author) reports

This is also somewhat an artifact of the way (my project) is structured, and the way I'm choosing to provide these parameters. Eventually I want to provide them as a secret, and have this just be a default option within (my project), but I need to demonstrate that this is valuable first, so, going with a slightly less elegant but cheaper to implement option to start with.

[pivotaljohn]

Currently reconsidering.

The currently supported way of doing this is to pair any newly-introduced template with an explicit data values overlay file that "declares" the additional keys (with reasonable defaults or zero values)... and then the -v and friends are available from the command-line.

[pivotaljohn]

As mentioned in #carvel ~Feb 2021...

In GitLab documentation on "Dynamic child pipelines", there's a pointer to ytt. It's easy to make the inference that one merely reference the many pre-defined variables in your templates and Bob's yer uncle. ... which would be true if we implemented this feature. (note the reliance on environment variables, in this case).

[erikgb]

In GitLab documentation on "Dynamic child pipelines", there's a pointer to ytt. It's easy to make the inference that one merely reference the many pre-defined variables in your templates and Bob's yer uncle. ... which would be true if we implemented this feature. (note the reliance on environment variables, in this case).

There is also an open GitLab issue to add an example of child/parent pipelines with ytt.