CI: enable new packaging tests using apkg

In order to solve #612 (closed):

• remove tests/packaging based on docker/py.test and ever-present .packaging - a total of 178 files / 1600 lines that achieved very little in an extremely redundant way
• remove distro/packaging based on vagrant/ansible (65 files / 800 lines)
• remove pkgtest using OBS from .gitlab.yml
• replace all above with a pkg stage using apkg

For each supported distro, build and install packages and run packaging tests:

apkg install --build-dep
apkg test --test-dep

This is using new apkg LXC images based on lxc-gitlab-runner.

TODO

• prepare apkg LXC images for all currently supported distros
• enable correct Knot DNS/build deps repos (probably script using python-distro which is already required by apkg)