[Merge] ~rcj/livecd-rootfs/+git/livecd-rootfs:feature/xenial_snap_bases into livecd-rootfs:ubuntu/xenial

[Robert C Jennings]

Built ubuntu-cpc images and tested that images with core or core18 based snaps build as expected.
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https://code.launchpad.net/~rcj/livecd-rootfs/+git/livecd-rootfs/+merge/382939
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~rcj/livecd-rootfs/+git/livecd-rootfs:feature/xenial_snap_bases into livecd-rootfs:ubuntu/xenial.