Add network check methods to Device and Workload classes

- Device subclasses should provide their own implementation.
    Default behaviour is to raise a `NotImplementedError`
  - Workload subclasses can set `requires_network` to `True` and
    network connectivity check will be performed during `setup()`

wlauto/core/device.py

@@ -426,6 +426,13 @@ class Device(Extension):
         """
         pass
 
+    def is_network_connected(self):
+        """
+        Checks if the device is connected to the internet
+
+        """
+        raise NotImplementedError()
+
     def __str__(self):
         return 'Device<{}>'.format(self.name)