Support for configuring update window for flatcar nodes

[ahmedwaleedmalik]

What would you like to be added? / User Story

As a user, I'd like the ability to configure the update window for flatcar-update-operator using the dashboard.

Acceptance criteria

• It's possible to configure the update window
• There should be some validation for the start and length fields in UpdateWindow.

References

• https://github.com/kubermatic/kubermatic/blob/master/pkg/apis/kubermatic/v1/cluster.go#L288
• https://github.com/flatcar-linux/flatcar-linux-update-operator/blob/master/doc/reboot-windows.md

[floreks]

API ref: https://github.com/kubermatic/kubermatic/blob/master/pkg/api/v1/types.go#L815