f+1 validators always approve (#2699)

* f+1 always approves

* guide

* grumbles

* grumbles

* fix test

* fix tests

* Update roadmap/implementers-guide/src/node/approval/approval-voting.md

Co-authored-by: Sergei Shulepov <sergei@parity.io>

Co-authored-by: Sergei Shulepov <sergei@parity.io>