Reintroduce l3 router/gateway settings in l3 agent

This partially reverts commits f37dc6e037450a1c6dd0d20e05f99185efa6124f, e969a6f69eefc722c6fcd5d41ddbd6c30d36ce5b

Issue rcbops/chef-cookbooks#933

(cherry picked from commit 571259236b88652a4eef2185c5542b238a652f63)