Easier tailing of log files

This creates a `/var/log/<component>/<component>_current.txt` symlink pointing at the current log file (and updates it on every log rotation). This gets the key benefit of https://github.com/Metaswitch/clearwater-infrastructure/issues/63, without all the upheaval of actually moving to logrotate.

Tested by running `tail -F /var/log/sprout/sprout_current.txt` and seeing continued output across the hour boundary.