CI: cross-project integration tests 🐞

Add new CI stage "integration" which is only run when $INTEGRATION
variable is set.

Add new testing images:

* debian-10, debian-10-full (previously python-current)
* fedora-33
* suse-15,
* ubuntu-18.04, ubuntu-20.04

Add new integration jobs on supported images for first projects:

* BIRD
* Knot DNS
* Knot Resolver

Jobs on images with known issues are allowed to fail for the time being.

Fixes: #24
4 jobs for kresd in 5 minutes and 13 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #536732
flake8

00:03:56

passed #536733
pylint

00:04:19

passed #536735
self

00:04:22

passed #536734
unit

00:05:13