refactor!: rewrite and cleanup kubenurse server code (postfinance#29)

* refactor!: rewrite and cleanup kubenurse server code

By using a package and multiple separate files the code is easier to
understand and test. A new /ready handler was added so we can configure
a readiness probe to allow seamless updates of kubenurse.

* build: update golangci-lint version

* build: update golangci-lint timeout, default is too short

* build: extract lint step and use go version 1.17

* feat: configure new readinessprobe in kustomize and helm templates

* fix: linter errors

* chore: cleanup, remove not needed WaitGroup

* refactor!: move pkg/kubediscovery to internal/kubediscovery

* refactor!: move pkg/checker to internal/servicecheck

* refactor!: incorporate pkg/metrics in internal/servicecheck

* refactor!: more refactorings to allow easier unit testing

* feat: more unit tests and coverage calculation in workflows

* docs: include ci and coverage badges in readme

* docs: fix coverage status URL

Co-authored-by: Clément Nussbaumer <[email protected]>

* chore: embed servicecheck.Result in /alive output for simplicity

Co-authored-by: Clément Nussbaumer <[email protected]>

chore: bump version to 1.4.1

chore: bump version to 1.4.0

CI: Use latest-ci image for CI testing

feat: CI improvements and RBAC fixes

docs: Add toleration example for master/control-plane

Merge pull request postfinance#16 from postfinance/feat/lint-and-ci

ci: setup github actions, configure golangci-lint and fix lint errors

Merge pull request postfinance#13 from postfinance/feat/discovery

exclude nodes which are not schedulable from neighbour checks

tag v1.2.0

update DaemonSet to v1.1.3