Add "Conflicts" to our lxc-docker package to make it more clear that we can't/shouldn't be installed alongside either "docker" or "docker.io"

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4206 from tianon/ubuntu-conflicts

Add "Conflicts" to our lxc-docker package