Request to allow use of system-files interface for canonical-livepatch

@domas hey.

I see canonical-livepatch is requesting read and write access to /etc/update-motd.d/99-livepatch-kernel-upgrade-required. I am +1 to grant such access since this file is clearly owned by canonical-livepatch. Can other @reviewers please vote?

Since write implies read, can you please update the snap declaration and leave the write access only? Can you please also rename the interface reference to better reflect the location of the config file being accessed (i.e. etc-config-motd or even etc-update-motd-d instead of config-motd only? If you check the system-files doc is what we usually request as part of this process.

Thanks,