Make writer_is_also_reader configurable for hostgroup-pairs (instead of globally)

[asivado]

The current implementation of mysql-monitor_writer_is_also_reader is global - you can either set it to true or false for all hostgroup-pairs (documentation here). We should use hostgroup attributes to implement it in a way that lets the user decide if they want to enable it or not on a per-hostgroup-pair level.