change radius.log to a append only log file

Summary:
disk_log's can not rotated with logrotate. Switch the log
file to append only log to allow logrotation.

Test Plan: build and run

Reviewers: skattelmann, druss

Reviewed By: druss

Subscribers: aschultz, druss, mkraemer

Maniphest Tasks: T579

Differential Revision: https://review.tpip.net/D1651