Buildout debug mode for slapos node software / instance

Expose buildout debugging capabilities in slapos node software and slapos node instance with a --buildout-debug command line option.

From buildout's command line reference:

-D Debug errors. If an error occurs, then the post-mortem debugger will be started. This is especially useful for debugging recipe problems.

This is to make it easier to debug instance profiles written as jinja templates.

This fix from buildout https://lab.nexedi.com/nexedi/slapos.buildout/merge_requests/17 is recommended, otherwise slapos node does not realize there was an error in buildout if that error is debugged.