add TD_AGENT_CONFIG_FILE to td-agent.init

Hi.

This pull-request is change to default of daemon startup option.
`start` and `configtest` is Implicitly using `/etc/td-agent/td-agent.conf`.

If the user wants to change the configuration file,
then it should wrote explicitly the configuration file as a variable.

However, I think this change has a big influence.
I want you to confirm it.

Thank you.