Refactor fetching retain ports when reconciling SecurityGroup

[zarvd]

What type of PR is this?

/kind bug

What this PR does / why we need it:

• Refactor the function that retrieves the ports for services using the same external IP.
• Reduce a call to fetch PublicIP.
• Add unit test cases.

Which issue(s) this PR fixes:

Fixes #6151

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Fix updating NSG rules when disabling floating IP 

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

[k8s-ci-robot]

Hi @zarvd. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[nilo19]

/ok-to-test

[nilo19]

Do you consider internal/testutil -> testutil/?

[zarvd]

Do you consider internal/testutil -> testutil/?

I am thinking not to export this pkg, and maybe some more

[nilo19]

Do you consider internal/testutil -> testutil/?

I am thinking not to export this pkg, and maybe some more

Then I think we should include this change within the pr.

[nilo19]

/lgtm

[zarvd]

/retest

[MartinForReal]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: MartinForReal, zarvd

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/cloud-provider-azure/blob/master/OWNERS)~~ [MartinForReal] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[MartinForReal]

/lgtm

[coveralls]

Changes unknown when pulling 4f1f24a7161acbce60cdb22f656a27d1a54e8a01 on zarvd:refactor/tidy-access-control into on kubernetes-sigs:master.

[zarvd]

/kind bug