Move Eventlogging logs underneath /srv, which has more free space

vanadium is seeing daily alerts about the disk space running low on
its root partition [1].
EventLogging writes its logs to the vanadium's root partition.
Since the EventLogging logs have been dialed back from retaining 90
days of data to 45 day some time back (bug 69029), and vanadium's /srv
mount has plenty of free space left anyways [2], it's about time to
solve the spcae problem for good and we hence now write EventLogging
logs into a directory underneath /srv.

[1] See for example 06:26:04 on
  http://bots.wmflabs.org/~wm-bot/logs/%23wikimedia-operations/20141111.txt

[2] Currently, /srv has 250GB free. All of EventLogging logs on
vanadium currently take ~70GB.

RT: 8063
Change-Id: I41f628acdad7506f0c6813648ce3b85cf216cb4b