[Merge] ~vorlon/ubuntu-seeds/+git/ubuntu:seed-lxd-installer into ~ubuntu-core-dev/ubuntu-seeds/+git/ubuntu:kinetic

[Paride Legovini]

A positive side effect of keeping lxd-installer installed is that lxd is always available on a system where ubuntu-server-minimal is installed, even if the snap gets removed.
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https://code.launchpad.net/~vorlon/ubuntu-seeds/+git/ubuntu/+merge/424327
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