Support VMSS in flex mode

This allows to use VMSS in both the uniform and flex mode.

For flex mode, we need to use "regular" VM APIs instead of VMSS APIs for some things. So on Init, we discover which mode the VMSS is setup with, and based on that use different APIs when talking to Azure.

• closes: #4 (closed)
• related: https://gitlab.com/gitlab-com/request-for-help/-/issues/2621