Fix regex_pattern for carbon-aggregator rules.

In its current implementation the regex_pattern would match any metric that starts with the given pattern. Consider the following rules:

```
aggregated.histogram.p99        (10) = avg hosts.*.histogram.p99
aggregated.histogram.p999       (10) = avg hosts.*.histogram.p999
```

Currently the second rule would be treated as expected but the first rule would lead to an aggegated metric `aggregated.histogram.p99` which would in fact be equivalent to `avgSeries(hosts.*.histogram.p99,hosts.*.histogram.p999)`.

This patch forces exact matches of the regex_pattern, not only prefix-matches.