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

[simao]

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.

[obfuscurity]

What happens if alias is unset?