[Merge] ~rbalint/livecd-rootfs:make-rprivate-before-umount into livecd-rootfs:ubuntu/master

[Balint Reczey]

Review: Resubmit

Tests are passing with --make-private and umounting without -R, too:
https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-disco-rbalint-scratch/disco/amd64/l/livecd-rootfs/20190205_021604_bb80e@/log.gz
-- 
https://code.launchpad.net/~rbalint/livecd-rootfs/+git/livecd-rootfs/+merge/362686
Your team Ubuntu Core Development Team is subscribed to branch livecd-rootfs:ubuntu/master.