FTS: incorrect index replicas distribution through server groups

Build: 7.0.0-5127

• Create cluster {kv, index, query},{kv, index, query},{fts},{fts},{fts},{fts}
• Make the following server groups: 

sg1:{kv, index, query},{fts},{fts}

sg2:{kv, index, query},{fts},{fts}

• Create default bucket, load data using cbworkloadgen

./cbworkloadgen -n 172.23.107.35:8091 -r 100 -i 100000 -b default -s 1024 -j -u Administrator -p password

• Create default fts index (partitions: 1, replicas: 1)
• Check partitions distribution through server groups.
• Both fts nodes from sg1 group hold 1 index partition, none of fts nodes from sg2 hold index partitions
•  Expected behavior: 1 fts node from each server group holds 1 partiton, another 2 fts nodes are empty.