Enhancement: Unify repository layout for all backends

Up to now the s3 backend used a special repository layout. We've decided to
unify the repository layout and implemented the default layout also for the s3
backend. For creating a new repository on s3 with the default layout, use
`restic -o s3.layout=default init`. For further commands the option is not
necessary any more, restic will automatically detect the correct layout to use.
A future version will switch to the default layout for new repositories.

https://github.com/restic/restic/pull/966
https://github.com/restic/restic/issues/965
