feat: Support existing load balancer backend address pool for agent nodes

[xuto2]

Reason for Change:
Azure Standard Load Balancer(SLB) introduces some fundamental difference to Azure Basic Load Balancer(BLB). Specifically, vmas/vmss behind Azure SLB don't automatically get internet access like BLB, and it's Azure's recommendation that we "always use outbound rules on a Standard public Load Balancer" (see https://docs.microsoft.com/en-us/azure/load-balancer/load-balancer-outbound-connections for details).

To support Azure SLB in AKS, AKS needs to maintain the outbound rules of the SLB (and the load balancer rules as well to workaround upstream issue when using SLB kubernetes/kubernetes#76691). So we need the ability to place AKS engine generated nodes (VMSS+VMAS) into an existing SLB's backend address pool.

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

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[jackfrancis]

@sylr Does this address any symptoms described in #1099?

[jackfrancis]

/azp run pr-e2e

[CecileRobertMichon]

/needs-rebase

[jackfrancis]

@xuto2 FYI I rebased on top of master/force-pushed to this branch, so you'll need to pull down for future dev

[CecileRobertMichon]

Squash commit message needs to be updated to use semantic message (or if a second commit is pushed the title is enough).

/hold

[jackfrancis]

/azp run pr-e2e

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[CecileRobertMichon]

two commits will use PR title as squash message so we're good now.

/hold cancel

[jackfrancis]

/azp run pr-e2e

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@5f86d3d). Click here to learn what that means.
The diff coverage is 100%.

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[CecileRobertMichon]

lgtm

[CecileRobertMichon]

/azp run pr-e2e

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[feiskyer]

LGTM

[jackfrancis]

@sylr nevermind I understand what this does, I have another possible fix for the lack of UDP outbound incoming shortly...

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉

[xuto2]

Thanks @jackfrancis @CecileRobertMichon @feiskyer !