Stage for 2015-spring-preview.
Enable audio controller. Add '-preview' as suffix to our image name.
Enable and write to the PACKER_LOG file.
Install an alsa package to support audio.
Disable call to etckeeper init. The etckeeper package does this during
postinst.
Unset BCE provisioner as etckeeper committer during clean-up.
Clean up logging. Do not invoke etckeeper when there are no changes to
/etc.