Feat: make stacker-bom discover implicit

[mikemccracken]

Is your feature request related to a problem? Please describe.

needing to change the run section of stacker-bom discover and be careful about timing when it is run and when things are cleaned up that it might require is more work for the stackerfile author than it needs to be.

As I understand it, the intent is to allow removing any package manager data from the final image for size/etc purposes, but because the bom discover step needs that data, we need to run the discovery before removing the data.

Describe the solution you'd like

it would be nice if adding bom generation to a stackerfile only meant adding a bom: section and did not require changes to the run section.

What ought to work is to say that if bom.generate is true, then stacker will do the bom discover step after the run section is done, and then we can add a flag to the bom section to control whether or not the package manager data gets cleaned up. By default it won't, to allow derived images to install packages.

Describe alternatives you've considered

No response

Additional context

No response