This updates the builder to store individual build logs in dedicated
files, one per build, named for the image and build id. Old logs are
automatically pruned. By default, they are stored in
/var/log/nodepool/builds, but this can be changed.
This removes the need to specially configure logging handler for the
image build logs.
Change-Id: Ia7415d2fbbb320f8eddc4e46c3a055414df5f997