ExtraRoute: fix _get_extra_routes_by_router_id()

router_id filter wasn't properly applied to the query
so the func returned all extra routes of all routers.

Closes-Bug: #1248219
Change-Id: I77733ee7994c8fccbbc966ccabd83b8fe4def8d5
Upstream-Review: https://review.openstack.org/#/c/55532/2
Backported-from: icehouse
Closes-rally-bug: DE1742