Improve error message for conflicting container name.

This changes the error message that is returned by the daemon when
a container-name already exists.

The old message suggests that containers can be renamed, which is
currently not possible.

To prevent confusion, the part "(or rename)" is removed from
the error-message.

Message before this change;

    FATA[0000] Error response from daemon: Conflict, The name foobar is already assigned to 728ac36fb0ab. You have to delete (or rename) that container to be able to assign foobar to a container again.

Message after this change;

    FATA[0000] Error response from daemon: Conflict, The name foobar is already assigned to 728ac36fb0ab. You have to delete that container to be able to assign foobar to a container again.

Relates to: https://github.com/docker/docker/issues/3036

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>