Ability to support multiple windows baselines / licenses

Missing feature description:
Currently, it is possible to upload multiple Windows images of the
same version specifying different names:


etc. This is useful for the different flavors of Windows or for
simply 2 very different Windows baselines. However, the Windows
licensing features of MAAS only supports a license key for
windows/win2016 (or one of the other few predefined names). There are
different license keys for example for Standard vs Datacenter
editions of Windows. Another potential use case is multiple tenants
provide their own license keys for Windows. The license key feature
in MAAS should be updated to allow adding license keys for any of the
images that are uploaded to it with custom names.