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adding support for VM name with extra Separator String

**What this PR does / why we need it**:
Allows the Azure VM name to have within their name the `_` character

**Special notes for your reviewer**:
This is the error I got before testing
```
May 07 21:55:14 kn-infra000000.<domainname> kubelet[39465]: I0507 21:55:14.462125   39465 azure_vmss.go:108] getVmssVM gets scaleSetName ("kn-infra_ss") and instanceID ("0") for node "kn-infra000000"
May 07 21:55:14 kn-infra000000.<domainname> kubelet[39465]: E0507 21:55:14.462147   39465 azure_vmss_cache.go:52] Failed to extract vmssVMName "kn-infra_ss_0"
May 07 21:55:14 kn-infra000000.<domainname> kubelet[39465]: F0507 21:55:14.462160   39465 kubelet.go:1349] Kubelet failed to get node info: failed to get external ID from cloud provider: not a vmss instance
```

**Release note**:
```release-note
Azure VMSS: support VM names to contain the `_` character
```