Add PowerMetric to Board

[majst01]

Sample output of cli for a bigtwin waiting machine:

Power:
api.PowerMetric{AverageConsumedWatts:56, IntervalInMin:5, MaxConsumedWatts:57, MinConsumedWatts:55}

and a n1-medium:

api.PowerMetric{AverageConsumedWatts:45, IntervalInMin:5, MaxConsumedWatts:45, MinConsumedWatts:45}

and a storage machine:

api.PowerMetric{AverageConsumedWatts:559, IntervalInMin:5, MaxConsumedWatts:559, MinConsumedWatts:559}

closes #53