Watch Action instead of polling Server Status

[fhofherr]

We have recevied an issue from one of our customers that their
autoscaler pod regularly crashes due to a nil pointer dereference panic.
Analyzing the code we found out that the autoscaler polls the server
status to find out if a server is running.

The Hetzner Cloud Go client is implemented in such a way that it does
not return an error if a resource could not be found. Instead it returns
nil for the error and the resource. Ususally this is not an issue.
However, in case the server creation fails the server gets deleted from
Hetzner Cloud. This in turn leads to nil being returned and the
abovementioned panic.

The Hetzner Cloud API implements a concept called Actions. Whenever a
long running process is triggered we return an object which can
be used to get information about the progress of the task. The Action
object reliably allows to detect if a server has been created and
provides access to any error that may have occured.

This commit replaces polling the server status with using the action
object.

[k8s-ci-robot]

Welcome @fhofherr!

It looks like this is your first PR to kubernetes/autoscaler 🎉. Please refer to our pull request process documentation to help your PR have a smooth ride to approval.

You will be prompted by a bot to use commands during the review process. Do not be afraid to follow the prompts! It is okay to experiment. Here is the bot commands documentation.

You can also check if kubernetes/autoscaler has its own contribution guidelines.

You may want to refer to our testing guide if you run into trouble with your tests not passing.

If you are having difficulty getting your pull request seen, please follow the recommended escalation practices. Also, for tips and tricks in the contribution process you may want to read the Kubernetes contributor cheat sheet. We want to make sure your contribution gets all the attention it needs!

Thank you, and welcome to Kubernetes. 😃

[blindpirate]

Thanks @fhofherr .

I've already built my custom CA image on top of this PR. It works perfectly and I no longer see crashing.

[mwielgus]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fhofherr, LKaemmerling, mwielgus

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• cluster-autoscaler/OWNERS [mwielgus]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment