Feature - Add filters for pushers

[dsaingre]

Background

The current version of PowerAPI support filters between the puller and the dispatcher, allowing to limit the report processed by the formula. However, we can't filter the report to be saved on the database by the pusher. This is an issue for instance with the filedb database that only save the last report processed by PowerAPI: when processing several targets, it does not make sense to have a record that correspond to a random target (we would like to have records for a fix target, that could later be used in a virtual machine with procfs sensor).

Proposition

Implement a filter for pushers. This filter could be parametrized through the formula configuration.

Related issues

#134