[workload] Allow specifying PodSetUpdates for a subset of PodSets

[trasc]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Allow an admission check controller to provide podSetUpdates for a podSet subset when needed.

Which issue(s) this PR fixes:

Fixes #2260

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Fix support for MPIJobs when using a ProvisioningRequest engine that applies updates only to worker templates.

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[trasc]

/assign @alculquicondor

[alculquicondor]

/approve
/hold for nits

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: alculquicondor, trasc

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:

• pkg/webhooks/OWNERS [alculquicondor]
• test/OWNERS [alculquicondor]

Approvers can indicate their approval by writing /approve in a comment
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[alculquicondor]

/release-note-edit

Fix support for MPIJobs when using a ProvisioningRequest engine that applies updates only to worker templates.

[mimowo]

/lgtm
/hold cancel
I believe all comments are addressed. If there are some remaining improvements we can follow up.

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 909d211236f73193915a9084b5e2e63186db885c

[tenzen-y]

/cherry-pick release-0.6

[k8s-infra-cherrypick-robot]

@tenzen-y: #2265 failed to apply on top of branch "release-0.6":

Applying: Allow specifying PodSetUpdates for subset of PodSets
Using index info to reconstruct a base tree...
M	pkg/webhooks/workload_webhook.go
M	pkg/webhooks/workload_webhook_test.go
M	test/integration/controller/jobs/mpijob/mpijob_controller_test.go
M	test/integration/webhook/workload_test.go
Falling back to patching base and 3-way merge...
Auto-merging test/integration/webhook/workload_test.go
CONFLICT (content): Merge conflict in test/integration/webhook/workload_test.go
Auto-merging test/integration/controller/jobs/mpijob/mpijob_controller_test.go
Auto-merging pkg/webhooks/workload_webhook_test.go
Auto-merging pkg/webhooks/workload_webhook.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Allow specifying PodSetUpdates for subset of PodSets
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherry-pick release-0.6

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-sigs/prow repository.

[tenzen-y]

@trasc Could you create a cherry-pick PR?