[Merge] ~patviafore/livecd-rootfs/+git/livecd-rootfs:use_ftpmaster_instead_of_archive_in_binary_hooks into livecd-rootfs:ubuntu/master

[Pat Viafore]

Pat Viafore has proposed merging ~patviafore/livecd-rootfs/+git/livecd-rootfs:use_ftpmaster_instead_of_archive_in_binary_hooks into livecd-rootfs:ubuntu/master.

Commit message:
Use LP supplied archives for binary hooks

Replace security.ubuntu.com and archive.ubuntu.com with LP supplied archives. 
These are faster, are more up-to-date, and what are already used for chroot hooks.

This saves off a copy of the sourceslist to restore when the setting up the mountpoint. 
This gets restored when the mountpoint is torn down; this is to prevent the LP 
archives from leaking into the build. 

If a binary hook modifies the sources.list, the build will fail by 
detecting differences to the file. The binary hook has two options to rectify this:

1) If the hook needs to modify the sources.list for build purposes only, 
they can regenerate the sha256.

2) If the hook needs to modify the sources.list to be used in the final image, 
they can edit the backup file generated by use_lp_archives_in_sourceslist. 
The filename is stored in the variable "MOUNTPOINT_BACKUP_SOURCES_LIST"

Requested reviews:
  Ubuntu Core Development Team (ubuntu-core-dev)

For more details, see:
https://code.launchpad.net/~patviafore/livecd-rootfs/+git/livecd-rootfs/+merge/383790
-- 
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~patviafore/livecd-rootfs/+git/livecd-rootfs:use_ftpmaster_instead_of_archive_in_binary_hooks into livecd-rootfs:ubuntu/master.
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