Since nexedi/erp5@6c6b77a0, this is now a safe
operation, in that a mistake while moving databases won't result in a loss of
the catalog if the zodb is empty.

This new implementation relies on
nexedi/erp5@6bb4c56e, which means that zopes
decide on their own to create the site at startup if there's none, and the
'erp5-bootstrap' service is removed from the root partition.

/reviewed-on nexedi/slapos!217