Can't test against Azurite because Azure host is hardcoded

[NathanBaulch]

I'd like to be able to test an Azure backend against https://github.com/Azure/Azurite running locally, however I quickly discovered that unlike the other cloud backends, Azure is hardcoded with blob.core.windows.net.

It's also a bit odd that the Azure backend uses the general https:// scheme and seems to get special treatment in vfssimple which calls azure.IsValidURI.

I think it'd be a good idea to make the Azure backend consistent with the other cloud backends by introducing an az:// scheme and letting the underlying client decide the target host. Speaking of clients, it would be also be good to get rid of azure.DefaultClient and simply use/mock the standard azure-sdk-for-go client.

[funkyshu]

I never liked the scheme as http. We built the backend intending to use azure blob for a specific client but when the deal fell through, so did the development on that backend. I've considered deprecating it. I'm open to any PR you have for this including changing the scheme.