Add custom Ubuntu 18.04 arm64 packer file

The default file in common-packer calls for the flavor
"lf-standard-1", which Akraino does not have in its cloud. By
switching to an available flavor, this should fix our broken packer
build.

Change-Id: I213167072395b0985b914776068ad502aa3c8209
Signed-off-by: Eric Ball <eball@linuxfoundation.org>