Allow incomplete fact selection

[d3vco]

Description

This change to the cartesian function allows a user to create a potential link without supplying values for every fact template.

If a fact template does not have a selected value, it is not replaced in the command. This is useful if the user has edited the command to remove a fact template. The change also allows for appropriate substitution of global facts (i.e. #{server}) when mixed with plugin facts.

fixes mitre/caldera#3030

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Unit test added.

Integration tested by adding to a local instance of Caldera and replicating some representative use cases.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

[elegantmoose]

Tested.