add supervisor_conf_template variable for main config file customization

[BahamutNeRo]

Description

Use "{{ supervisor_conf_template }}" variable instead of hardcoded template "supervisord.conf.j2" for more main config versatility.

For example, currently there is no way to add settings like: minfds = XXXXX chmod = XXXX to supervisor.conf w/o role editting.

Thx!