Load `{data_dir}/configs/cluster.hocon` when generating node boot config.

Logging related config changes made from the dashboard are persisted in `{data_dir}/configs/cluster.hocon`.
Prior to this change, it only takes `etc/emqx.conf` to generate the boot config (including the logger part),
then `{data_dir}/configs/cluster.hocon` is loaded to reconfigure the logger after boot is complete.

This late reconfigure may cause some log segment files to be lost.

Now `{data_dir}/configs/cluster.hocon` and `etc/emqx.conf` are both loaded (`emqx.conf` overlaying on top)
to generate boot config.