Improve OS detection when VM has no osProfile (#197)

We have VM without OS profile (checked on https://resources.azure.com/) and they appear as "unknown" os :-/

This appear to be a known issue: https://support.microsoft.com/en-ph/help/4018140/computer-names-of-specialized-virtual-machines-are-missing-or-blank-in

Co-authored-by: Fred-sun <37327967+Fred-sun@users.noreply.github.com>
Co-authored-by: haiyuan_zhang <haiyuan.zhang1982@gmail.com>

plugins/inventory/azure_rm.py

@@ -532,6 +532,12 @@ def hostvars(self):
                 system = 'linux'
             if 'windowsConfiguration' in self._vm_model['properties']['osProfile']:
                 system = 'windows'
+        else:
+            osType = self._vm_model['properties']['storageProfile']['osDisk']['osType']
+            if osType == 'Linux':
+                system = 'linux'
+            if osType == 'Windows':
+                system = 'windows'
 
         new_hostvars = dict(
             public_ipv4_addresses=[],