slapgrid: Add --force-stop option

Add a --force-stop option to instance processing, as in: slapos node instance --force-stop

This option affects how started partition are processed:

• buildout is processed normally
• the timestamp is ignored and not updated
• the services are stopped instead of started
• no promises are run
• no report is sent to master

This is useful when we want to do as much work as possible in advance (run buildout, create configuration files, etc), without actually starting the instance, e.g. for resilient clones.