Expose Azure useImageGallery parameter in the MachineSets() call

As a downstream consumer of the installer (as a library), I want to be able to choose whether or not the image gallery is used when creating machinesets on Azure so that I can achieve backwards compatibility with pre-4.12

4.12+

always

1. Try to generate machinesets in pre-4.12 environment
2. Lament as the installer automatically uses image gallery regardless

Installer attempts to guess whether to use image gallery

I should be able to choose myself