out_file plugin should allow customizing permissions

When using fluentd to write logs to disk using `out_fule` plugin, for certain sensitive logs, it would be good to be able to restrict the ownership and access of the files. At the moment, all output files are owned by the user fluentd runs as, and with default umask, so probably world-readable.

Ideally we'd be to be able to configure for each file, the owner, group, and standard Unix permissions (eg 0600).

Environment information - fluentd 0.12, in docker container on CentOS 7.5.