Also check presence of trigger_file on master after promotion when
pg_is_in_recovery() = false and if it is there - remove it.
Plus check presence of trigger_file on a new slave (after running
pg_basebackup) and if it is there - also remove it.
Refactored query and is_healthiest_node methods
query method wont retry more then 3 times if OperationalError raised
is_healthiest_node takes into account that one of the nodes could be
still a master