Architectures supported by Travis due not match 1 to 1 to the
architectures supported by snapcraft.io and launchpad. To support as
many architectures as possible, deploys to the snap store should be done
via a simulated Github webhook. An additional secret will need to be
added to Travis-ci
Snapcraft 4.0 complains about running LXD builds under sudo. So instead
add the CI user to the "lxd" group and use sg to switch to that
membership.
Resolves: #12712
