Add feature support for Azure VMSS mix size

[dearxieyali]

Which component are you using?:
cluster-autoscaler,

Is your feature request designed to solve a problem? If so describe the problem this feature should solve.:
Yes, In my environment , each app is deployed in seperated nodepool. when scaling out nodes in sepcific node pool , the scale out failed because of VM quotas limitation or other reasons. I need the autoscaler to scale out another VM size to complete the scale out.
Azure VMSS can support mix vm size , but autoscaler log show "instance type Mix not supported" in log

Describe the solution you'd like.:
So could Azure auto scaler support the Azure VMSS mix VM size.

Describe any alternative solutions you've considered.:

Additional context.:

[adrianmoisey]

/area cluster-autoscaler

[jackfrancis]

@dearxieyali you'll want to look into using VMSS Flex pools in Azure. That can be enabled in CA when using the azure cloudprovider via the AZURE_ENABLE_VMSS_FLEX environment variable, documented here:

• https://github.com/kubernetes/autoscaler/blob/master/cluster-autoscaler/cloudprovider/azure/README.md#vmss-deployment