Enhance set_testing-240-t to iterate through alll the reader hostgroups

[renecannao]

This is an internal development issue.

set_testing-240-t creates a new hostgroup 101 with all the reader hostgroups (all hostgroups ending with 1) .
This works well if using different backend versions, but doesn't work well if mixing MySQL with MariaDB or Galera .
As our testing infra becomes more complex with multiple backend types/clusters present at the same time,
set_testing-240-t stops working properly.

A possible solution to this problem is for set_testing-240-t to detects the number of reader hostgroups (before starting any test) and iterate through all of them when creating hostgroup 101 .
In other words: set_testing-240-t (whole or partially) will independently run on each reader hostgroups.