CI: cross-project integration tests 馃悶
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
Add CI schedule for integration tests.
Fixes: #24 (closed)