understand log_directory and shared_part_list

These are options passed by test node, so that tests:

• can expose their logs while they are running ( see https://lab.nexedi.com/nexedi/erp5/commit/34601253 )
• can use existing shared parts if they need to install software in a recursive slapos ( see https://lab.nexedi.com/nexedi/erp5/commit/35768fa2 )