exoscale provider: Update cluster autoscaler documentation

[PhilippeChepy]

Which component this PR applies to?

This PR applies to the cluster-autoscaler, in the Exoscale public cloud.
This is a documentation refresh.

What type of PR is this?

/kind documentation

What this PR does / why we need it:

Update the cluster autoscaler README for the Exoscale provider:

• Change the examples deployments to use upstream container image instead.
• Update the control-plane deployment example (Fixes Cluster Autoscaler for Exoscale - FailedScheduling #4782)
• Document IAM operations an Exoscale API key needs in order to make the CAS works correctly.

Which issue(s) this PR fixes:

Fixes #4782

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

Welcome @PhilippeChepy!

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. 😃

[PhilippeChepy]

/assign @aleksandra-malinowska

[mwielgus]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mwielgus, PhilippeChepy

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