feat: update VMSS node pools

[jackfrancis]

Reason for Change:

This PR introduces the new CLI feature aks-engine update --node-pool <node pool name> to update VMSS node pools on existing cluster via AKS Engine ARM template generation.

tl;dr With a new version of AKS Engine compared to the version used to build your cluster, and/or with changes to the agentPoolProfiles definition in the generated apimodel.json output, you use aks-engine update to essentially push a new VMSS model to your node pool. Subsequent scale out operations against that VMSS will therefore use the updated VMSS model.

Some real-world examples:

• change the VM SKU
• change the OS distro
• change other AKS Engine-exposed OS configuration (e.g., sysctld)

In practice the implementation essentially re-uses aks-engine scale, giving that preëxisting command the right inputs to do an ARM template deployment against an existing VMSS with no change to the Count value.

Issue Fixed:

Requirements:

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

Notes:

[codecov]

Codecov Report

Merging #3830 into master will decrease coverage by 0.27%.
The diff coverage is 27.04%.

@@            Coverage Diff             @@
##           master    #3830      +/-   ##
==========================================
- Coverage   73.20%   72.92%   -0.28%     
==========================================
  Files         148      149       +1     
  Lines       25428    25579     +151     
==========================================
+ Hits        18614    18654      +40     
- Misses       5679     5789     +110     
- Partials     1135     1136       +1     

Impacted Files	Coverage Δ
cmd/update.go	24.28% <24.28%> (ø)
cmd/scale.go	12.69% <44.44%> (+0.99%)	⬆️
cmd/root.go	60.52% <100.00%> (+0.20%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 77fb788...901cb75. Read the comment docs.

[jackfrancis]

Worth mentioning: detailed documentation will follow this PR (as part of a larger set of improved documentation around all CLI functionality)

[mboersma]

This makes sense to me and should help users quite a bit.

We should update the aks-engine help example output in docs/community/developer-guide.md to include this command.

[jackfrancis]

This makes sense to me and should help users quite a bit.

We should update the aks-engine help example output in docs/community/developer-guide.md to include this command.

Done

[mboersma]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis, mboersma

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:

• OWNERS [jackfrancis,mboersma]

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