Update strategy used to reuse CPUs from init containers in CPUManager

[klueska]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind api-change

/kind bug

/kind cleanup
/kind deprecation
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
With the old strategy, it was possible for an init container to end up
running without some of its CPUs being exclusive if it requested more
guaranteed CPUs than the sum of all guaranteed CPUs requested by app
containers. Unfortunately, this case was not caught by our unit tests
because they didn't validate the state of the defaultCPUSet to ensure
there was no overlap with CPUs assigned to containers. This patch
updates the strategy to reuse the CPUs assigned to init containers
across into app containers, while avoiding this edge case. It also
updates the unit tests to now catch this type of error in the future.

Does this PR introduce a user-facing change?:

Fixes regression in CPUManager that had the (rare) possibility to release exclusive CPUs in app containers inherited from init containers. 

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: klueska

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Needs approval from an approver in each of these files:

• pkg/kubelet/cm/cpumanager/OWNERS [klueska]

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

[mattjmcnaughton]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: klueska

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/kubelet/cm/cpumanager/OWNERS](https://github.com/kubernetes/kubernetes/blob/master/pkg/kubelet/cm/cpumanager/OWNERS)~ [klueska]

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

/assign @sjenning

@klueska my apologies if I'm missing something obvious here... but do you know why its counting you as a "approver"? From the OWNERS file, it looks like you're a reviewer (https://github.com/kubernetes/kubernetes/blob/master/pkg/kubelet/cm/cpumanager/OWNERS)? Trying to figure out if we need to report a bug to the k8s-ci-robot folks :)

[klueska]

@klueska my apologies if I'm missing something obvious here... but do you know why its counting you as a "approver"? From the OWNERS file, it looks like you're a reviewer

@mattjmcnaughton I am an approver in pkg/kubelet/cm which makes me an approver here as well by inheritance

[mattjmcnaughton]

On Fri, Apr 24, 2020 at 07:41:09PM -0700, Kevin Klues wrote: > @klueska my apologies if I'm missing something obvious here... but do you know why its counting you as a "approver"? From the `OWNERS` file, it looks like you're a reviewer @mattjmcnaughton I am an approver in `pkg/kubelet/cm` which makes me an approver here as well by inheritance -- You are receiving this because you were mentioned. Reply to this email directly or view it on GitHub: #90419 (comment)

Ahhh gotcha :) ok, perfect :)

[nolancon]

/lgtm