backport project filtering for replication plugin

gerrit 2.8's replication plugin offers the possibility to filter replication configs by project name. without this, configuration has to be done via groups, which leads to an utter mess, especially if mirroring to multiple different layouts (i'm about to add github now, and we can't use the scheme we use on gitorious).

the change is 0b9811b3bcd13e9d858d341935e660117c735569 and applies cleanly on top of 2.7. the problem is that i don't know how to do it logistically ... we have no repository for the plugin yet.