[pve-devel] add support for Ubuntu 18.10 (Cosmic Cuttlefish)

[Thomas Lamprecht]

Adds support for Ubuntu 18.10 for creating it via dab and starting it through
pve-container. Also add it to dab-pve-appliances even if it's not finally
released, the dab.conf will stay the same and we can upload it once it got
officially released.

Did some simple test with a current build (using dab with this patches).
Privileged and unprivileged works just fine, at least when doing some simple
create, start connect to CT with SSH and do `apt update` tests