Issue #1259 Fix topology-related API endpoints for group replication setups

[ejortegau]

Related issue: #1259

Description

This PR improves methods associated with serving the /topology.* endpoints so that replication groups are properly represented. For example, for a topology such as the one shown below:

the topology representation is now such that orchestrator-client shows this:

[ 10430 ] user@sabre ~/go/src/github.com/openark/orchestrator (master) % ➜  ./resources/bin/orchestrator-client -c topology -i sabre:5000
sabre:5020       [0s,ok,8.0.22,rw,ROW,>>,GTID]
+ sabre:5000     [0s,ok,8.0.22,rw,ROW,>>,GTID]
  ‡ sabre:5001 [0s,ok,8.0.22,ro,ROW,>>,GTID]
  ‡ sabre:5002 [0s,ok,8.0.22,ro,ROW,>>,GTID]
    + sabre:5010 [0s,ok,8.0.22,rw,ROW,>>,GTID]
+ sabre:5022     [0s,ok,8.0.22,rw,ROW,>>,GTID]
  + sabre:5021   [0s,ok,8.0.22,rw,ROW,>>,GTID]

Notice that we use the double dagger character ‡ to distinguish group replication from classical replication.