Tolerate partial API discovery

[rfranzke]

What type of PR is this?
/kind bug

What this PR does / why we need it:
The API discovery is now tolerating a partial discovery failure, e.g. caused due to unavailable API groups.

Which issue(s) this PR fixes:
Fixes #35

Special notes for your reviewer:
I added a unit test that simulates the behaviour and tested it successfully in a real-world scenario. If you like you can use rfranzke/cpvpa-amd64:v0.8.2-1-g5754e13 Docker image for testing.

Does this PR introduce a user-facing change?:

The `cluster-proportional-vertical-autoscaler` is no longer crashing if it cannot fully discover the API. Instead, it tolerates a partial discovery as long as its relevant API groups are discoverable.

[k8s-ci-robot]

Welcome @rfranzke!

It looks like this is your first PR to kubernetes-sigs/cluster-proportional-vertical-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-sigs/cluster-proportional-vertical-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. 😃

[rfranzke]

/assign @MrHohn @ialidzhikov

[k8s-ci-robot]

@rfranzke: GitHub didn't allow me to assign the following users: ialidzhikov.

Note that only kubernetes-sigs members, repo collaborators and people who have commented on this issue/PR can be assigned. Additionally, issues/PRs can only have 10 assignees at the same time.
For more information please see the contributor guide

In response to this:

/assign @MrHohn @ialidzhikov

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[MrHohn]

Thanks! This is cleaner than I originally thought, LGTM with minor comments.
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: MrHohn, rfranzke

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:

• OWNERS [MrHohn]

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

[MrHohn]

Thanks!
/lgtm

[rfranzke]

Great! Thanks for merging! It would be nice if you could release a new version with this fix :)

[MrHohn]

Yep, will try to publish a new release this week. Opened #37 for that.

[rfranzke]

Thank you @MrHohn!