[query] fix constantLine() to return 3 steps, just like Graphite

[teddywahle]

Here is the Graphite source:

def constantLine(requestContext, value):
  name = "constantLine(%s)" % str(value)
  start = int(epoch( requestContext['startTime'] ) )
  end = int(epoch( requestContext['endTime'] ) )
  step = int((end - start) / 2.0)
  series = TimeSeries(str(value), start, end, step, [value, value, value], xFilesFactor=requestContext.get('xFilesFactor'))
  series.pathExpression = name
  return [series]

[robskillington]

LGTM