Add recreate_containers method to service
Implement UserError __str__ method
Be sure to test that recreate_containers updates config
Support volumes in config with an unspecified host path
service.recreate_containers() no longer removes the old containers

We need to keep them around until the new ones have been started.
Remove project.create_containers(), revamp project.recreate_containers()

`recreate_containers` now returns two lists of old+new containers, along
with their services.
Rework 'fig up' to use recreate_containers()
Reduce stop() timeout when recreating containers
Refactor recreate_containers() in preparation for smart name-preserving logic
Use an anonymous intermediate container so that when recreating containers, suffixes always start from 1
Quieter log output when recreating

Moved log stuff to Service, which I think makes more sense anyway.
Maybe.
Fix test regression
Update README for new 'fig up' behaviour