Refactor the way server_id is set for replicas

The simple way is to add '--server-id 2' after the name of the image of
the container. But GHA doesn't let us do that. The idea of mount a file
from our repo doesn't work because the repo is check out later in the
workflow and I failed to find a pre-job hook. Then I realized that this
MySQL option is dynamic! So we will set that in the test target!

tests/integration/targets/setup_mysql/replica1/mysql.cnf

@@ -1,4 +0,0 @@
-[mysqld]
-skip-host-cache
-skip-name-resolve
-server_id = 2

tests/integration/targets/setup_mysql/replica2/mysql.cnf

@@ -1,4 +0,0 @@
-[mysqld]
-skip-host-cache
-skip-name-resolve
-server_id = 3