platform/api/azure: add OSImage struct
platform/api/azure: add OSImage struct

this is imported from
github.com/Azure/azure-sdk-for-go/management/osimage where the field
ordering is incorrect for posting to the azure API.

dm0-/mantle,dm0-/mantle,marineam/mantle,glevand/coreos--mantle,quantum/coreos-mantle,marineam/mantle,quantum/coreos-mantle,glevand/coreos--mantle