use metric alias to find metrics instead of relying on target attribute

Sometimes graphite changes the target attribute it returns, so tasseo should not rely on it to find the data points.

This PR makes tasseo use the alias, which I think it is cleaner.

For example, if you try to use the following tasseo.metric.target:

```
scaleToSeconds(timeShift(stats.timers.rails.requests.count, '-30seconds'),1))
```

Graphite will return something like (**notice the quotes**):

```
scaleToSeconds(timeShift(stats.timers.rails.requests.count, -30seconds),1))
```

So it won't work, because tasseo can't find this metric in the response using the target it sent.

With this PR, graphite will always return `target: <alias>` so we can use `metric.alias` to find the data points in `metricResults`.