[FEATURE_REQUEST] Add Targeted global.user to collateral user

[TechDufus]

Is your feature request related to a problem? If so, please describe.

If you actually are using this with multiple FLUXNODE users (local linux user running flux), there is currently no flexibility to deviate from the value in global.user in user.yml.

Describe the solution you'd like

Allow specifying a user or flux_user value within the user collateral section (same level as zelid) so you can differentiate different flux users with different zelid's, collateral, etc...

This is useful if you are managing nodes for others, and they have other users specified.

Priority

Low (Nice-to-have)

Is this something you would be keen to implement

None