Rename the overlay storage driver

so that docker is started with `docker -d -s overlay` instead of `docker -d -s overlayfs`

Signed-off-by: Lénaïc Huard <lhuard@amadeus.com>
Remove the last references to overlayfs

This only renames docker internal structures.
It has no impact on the end-user.

Signed-off-by: Lénaïc Huard <lhuard@amadeus.com>