Tidy base image by removing /var/log files

diskimage-builder/elements/base/cleanup.d/99-tidy-logs added
to tidy up (remove) initial logs files on disk images.

Makes for slightly smaller images, and easier times tracing post
boot issues.

Change-Id: Iee98b5d818e080bcd9635482bc29349a71938087
Merge "Tidy base image by removing /var/log files"