NetworkPowerPort: make index optional (default 0)

Smart plugs and similar single-port network-controlled devices shouldn't
require the exporter config to include the 'index' argument. Make it
optional by including a default of 0 in NetworkPowerPort.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>

Author: threexc

labgrid/resource/power.py

@@ -12,11 +12,11 @@ class NetworkPowerPort(Resource):
     Args:
         model (str): model of the external power switch
         host (str): host to connect to
-        index (str): index of the power port on the external switch
+        index (str): optional, index of the power port on the external switch
     """
     model = attr.ib(validator=attr.validators.instance_of(str))
     host = attr.ib(validator=attr.validators.instance_of(str))
-    index = attr.ib(validator=attr.validators.instance_of(str),
+    index = attr.ib(default="0", validator=attr.validators.instance_of(str),
                     converter=lambda x: str(int(x)))