chore: use Standard LoadBalancer as default

[jackfrancis]

Reason for Change:

This PR changes default cluster configuration enforcement so that Standard is the LoadBalancer SKU type if the user does not declare a loadBalancerSku value in the kubernetesConfig configuration object.

"Basic" LoadBalancer configuration is no longer appropriate as a default for cluster configurations, as it precludes running LoadBalancer-backed Kubernetes services behind multiple node pool-enabled clusters.

Folks who want to continue using the legacy "Basic" LoadBalancer type will have to explicitly declare so in their api model:

{
    "apiVersion": "vlabs",
    "properties": {
	"orchestratorProfile": {
		"orchestratorType": "Kubernetes",
			"kubernetesConfig": {
				"loadBalancerSku": "Basic",
...

Issue Fixed:

Fixes #2952

Requirements:

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

Notes:

[jackfrancis]

@craiglpeters @ritazh FYI

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis

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[codecov]

Codecov Report

Merging #2998 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2998      +/-   ##
==========================================
- Coverage   72.12%   72.11%   -0.02%     
==========================================
  Files         141      141              
  Lines       26257    26250       -7     
==========================================
- Hits        18937    18929       -8     
- Misses       6227     6228       +1     
  Partials     1093     1093              

Impacted Files	Coverage Δ
pkg/api/defaults.go	92.20% <100.00%> (-0.09%)	⬇️
pkg/api/mocks.go	100.00% <100.00%> (ø)
pkg/api/types.go	94.26% <0.00%> (-0.90%)	⬇️
pkg/operations/kubernetesupgrade/upgrader.go	62.45% <0.00%> (+1.38%)	⬆️

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[mboersma]

/lgtm