Labs: allow for growing volumes

Also update role::labs::biglog to extend /var/log rather than
create a new volume (necessary for the new images).  Images
using the older image but that have not yet created a /var/log
will need to do so by hand.

Change-Id: I00abd321f3b1c06ce7a563bd3ace28bb9b3bd2d6
Merge "Labs: allow for growing volumes" into production