Remove logs_processor from root group and add it to syslog group

Since making changes to the CDN log rotation, the logs are now owned
by syslog.syslog and logs_processor can't read them. Adding logs_processor
to the syslog group should solve this. There's no reason for this user
to be in the root group either.
Merge pull request #159 from alphagov/change_logs_processor_groups

Remove logs_processor from root group and add it to syslog group