Add variable to have apt mount root read write

In cases where you have a read only root filesystem a snippet like this is
required to ensure apt is able to run correctly.

Obviously if /usr is on another filesystem that would need a fragment too but I
don't have a setup like that to test with.

The fragment originates at https://wiki.debian.org/ReadonlyRoot#Make_apt-get_remount_.2F_if_needed