- Closes apple#150, apple#394.
- Introduces `AttachmentConfiguration` type so that
  we can add key-value options to `--network` in the
  future.
- Eliminates redundant `ContainersService.Item` type.
- Since we now ensure at ContainersService that hostnames
  will not conflict, the network helper IP allocator
  now simply provides the existing IP if for an allocation
  on an existing hostname, which should handle (in an
  eventually consistent way) the case where a container
  fails to deallocate an IP on shutdown.