Refactor Azure CLI calls to API calls

[benc-uk]

The function GetVMIdentities() in azure/cli.go needs rewriting to call the Azure API rather than use the CLI

Reasons

• Eventually we many be able to remove dependence on Azure CLI entirely

Notes / Considerations

None

Outcomes

• New function getVM(vmName string, resGroup string) in vm.go to call the API and get the VM details,
• GetVMIdentities calls this new function

[sebastus]

There are a number of other functions that rely on az cli, notably those in pkg/testing/cli.go

[benc-uk]

I'm less worried about tests, it's runtime dependency I'd like to minimise
The ultimate prize would be to remove any reliance on the Azure CLI, but that would involve handling our own auth, getting tokens etc, which is long way off, but no reason to make things harder for ourselves.