Refs #37034 - Use apt-key add on Ubuntu 16.04

• On Ubuntu 16.04, which is supported by Canonical until 2026, you
  cannot place a GPG public key in "/etc/apt/trusted.gpg.d/" and use it
  to verify the signature of "Release" files.
• EoL Ubuntu 16.04: https://ubuntu.com/16-04
• Ubuntu 14.04 is no longer supported by Canonical; Ubuntu 18.04 and all
  supported Debian versions can handle GPG public keys in
  "/etc/apt/trusted.gpg.d/".
• By default, "curl" is not installed on Debian and Ubuntu.
• By default, "gpg" is not installed on all supported Debian and Ubuntu
  versions. On Ubuntu 16.04, the package for "gpg" is called "gnupg".
  However, all supported Debian and Ubuntu releases allow installing
  "gnupg" instead of "gpg".
• Test on Ubuntu 16.04 in a container (ignore the repository and package;
  this is just to ensure that the package installation works):

FROM ubuntu:xenial

RUN apt-get update && \
apt-get install -y ca-certificates curl gnupg

RUN curl --silent --show-error https://oss.atix.de/atix_gpg.pub | gpg --dearmor > /etc/apt/trusted.gpg.d/client.gpg && \
echo "deb http://oss.atix.de/Ubuntu20LTS/ stable main" > /etc/apt/sources.list.d/client.list && \
apt-get update && \
apt-get install -y apt-transport-katello