Remove redundant code from GCE's `tasks.apt`

GCE's `tasks.apt.SetPackageRepositories` was adding duplicated entries
to apt `info.source_lists`, even ignoring the package mirror specified
on manifest.
Move `AddBackports` to `common.tasks.apt`

The `AddBackports` task is needed by many different plugins, so is
better if it is available as a common task.

This closes #96.