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 slapos.buildout!17 is recommended, otherwise slapos node does not realize there was an error in buildout if that error is debugged.

/reviewed-on !51

This is helpfull when you need to include "as after installation" scripts, where things move foward with no questions asked.

Introduce a --buildout-debug option to slapos node software and slapos
node instance, that will run buildout with -D flag.

Add a `debug` argument, False by default, which if true keeps
interactive stdin and stdout.

slapos/grid/SlapObject.py: If there is .shared file in software directory, don't upload binary cache. It is the mark of shared software.

/reviewed-on nexedi/slapos.core!50

  This simplifies 2 pages on the tutorials.

On the subscription process, we would like the user (as anonymous), be able to pay before Join.

  This is a rename of slapos_trial_request into subscription request. This Bt5 aims to handle
  indirect requests (before get a Hosting Subscription) and it should contains approval system.

  Set preferred_cloud_contract as true (required, to generate initial payment)
  there are only a single packing list generated as the API usage packacking lists are gone.

The computation of the API usage is been removed. There is no reason to invoice the API usage anymore.

The consumption should be based on computer resource consumption or by time based subscriptions.