feat: make vmType optional

[feiskyer]

What type of PR is this?

/kind feature

What this PR does / why we need it:

feat: make vmType optional

vmType's default value is changed from 'standard' to 'vmss' since v1.28.

Which issue(s) this PR fixes:

Fixes #3099

Special notes for your reviewer:

Does this PR introduce a user-facing change?

vmType's default value is changed from 'standard' to 'vmss' since v1.28.

action required: if your cluster is not using any VMSS instances and 'vmType' is not configured in the cloud provider config file, please add vmType=standard in the config file before upgrading to v1.28.

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

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: feiskyer

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[lzhecheng]

/lgtm