feat: add seccompProfile

[joebowbeer]

Add/update securityContext configs:

• seccompProfile.type RuntimeDefault is needed for compliance with restricted policies in Kubernetes 1.19+
• Updated comment in metrics Deployment and README regarding readOnlyRootFilesystem

Checklist

• Commits are signed with Developer Certificate of Origin (DCO - learn more)
• A PR is opened to update KEDA core (repo) (if applicable, ie. when deployment manifests are modified)
• README is updated with new configuration values (if applicable)

Fixes #306
Fixes #301

[joebowbeer]

NOTE: seccompProfile is only supported in Kubernetes 1.19+

This is causing the 1.17 and 1.18 tests to fail.

[tomkerkhove]

It is best to use Helm operators to check if the feature is available to avoid this, can you take a look at this? This is typically done through the API version

[joebowbeer]

@tomkerkhove wrote:

It is best to use Helm operators to check if the feature is available to avoid this, can you take a look at this? This is typically done through the API version

I think the options are:

1. Add code to the templates to conditionally remove these elements from the replacement
2. Comment-out these elements in values until 1.19 is the minimum supported version

[tomkerkhove]

Add code to the templates to conditionally remove these elements from the replacement

I think that's the best approach

[joebowbeer]

@tomkerkhove PTAL

Since seccompProfile requires Kubernetes 1.19+ (otherwise validation fails), I think this is the best that should be done until the minimum version is raised from 1.17 to 1.19.

While it should be possible to finagle the values here, it is not possible in keda/config (kedacore/keda#3562) and I think it's better to keep the two in sync.

[tomkerkhove]

Let's make sure the change is being applied because nobody will notice the comment.

As per https://github.com/kedacore/governance/blob/main/SUPPORT.md#kubernetes-support I'll enforce Kubernetes v1.20.

[joebowbeer]

@tomkerkhove PTAL

I restored the seccompProfile changes.

[tomkerkhove]

LGTM, thanks!