v0.8.0-rc1

This is a pre-release for upcoming v0.8.0 release, targeting early adopters for testing, as this release includes a large refactor of operator implementation. Changelog below:

Added

• Added golangci-lint and codespell CI jobs using GitHub Actions.
• Added CI for vendor directory, generated files, Go modules tidiness, Dockerfile etc.
• Added missing documentation for all packages.
• Added integration tests for pkg/updateengine.
• Added unit tests for pkg/operator.

Changed

• Updated used Go version to 1.16 and Alpine version for container images to 3.14.
• Container images will be now published to ghcr.io/flatcar-linux/flatcar-linux-update-operator instead of
  quay.io/kinvolk/flatcar-linux-update-operator.
• update-agent now use a dedicated Service Account with lower privileges than the operator.
• update-agent now use a dedicated Pod Security Policy.
• update-operator now use more strict Pod Security Policy, as it no longer needs permissions
  for agent related resources.
• Flatcar Linux references has been changed to Flatcar Container Linux.
• Migrated from glog to klog,
  which is a recommended logger for Kubernetes operators.
• pkg/drain has been integrated into pkg/k8sutil.
• Errors returned from operator and agent runs are now properly wrapped and can be unwrapped by the user.
• Remaining CoreOS references has been replaced with Flatcar.
• pkg/k8sutil.GetVersionInfo() will now return an error when /etc/flatcar/update.conf file is present,
  but not readable.
• Marking node as Unschedulable should now be more robust on frequent Node object updates.
• Renamed Go module from github.com/kinvolk/flatcar-linux-update-operator to github.com/flatcar-linux/flatcar-linux-update-operator, following the move of repository into new organization.

Fixed

• Fixed various typos found by codespell.
• Operator will no longer leak leader election goroutine, it will be now shut down when stop channel gets closed.
• Operator will now internally gracefully return an error when leader election is lost, so process can be restarted
  to start a new leader election.

Removed

• Removed no longer used Travis CI, Jenkins configuration files and build scripts.
• Removed AttemptUpdate(), GetStatus() and RebootNeededSignal() functions from pkg/updateengine package.
• Removed deprecated --manage-agent, --agent-image-repo and --analytics flags from update-operator.
• Deprecated operator functionality to manage agent DaemonSet has been removed. Agent DaemonSet must be now deployed separately.