azure_rm_virtualmachine: Protect against no diskSizeGB (#185)

Disks `diskSizeGB` is not reported if the Virtual Machine is
deallocated.  Trying to fetch it is causing a `KeyError`. This guards
against that KeyError and then nulls out the key so it isn't set
incorrectly in the update call.

plugins/modules/azure_rm_virtualmachine.py

@@ -1593,7 +1593,7 @@ def exec_module(self, **kwargs):
                                 vhd=data_disk_vhd,
                                 caching=data_disk.get('caching'),
                                 create_option=data_disk.get('createOption'),
-                                disk_size_gb=int(data_disk['diskSizeGB']),
+                                disk_size_gb=int(data_disk.get('diskSizeGB', 0)) or None,
                                 managed_disk=data_disk_managed_disk,
                             ))
                         vm_resource.storage_profile.data_disks = data_disks