util/packer: Support installing a custom flynn-host deb

Signed-off-by: Lewis Marshall <lewis@lmars.net>
util/packer: Support vagrant headless mode

Signed-off-by: Lewis Marshall <lewis@lmars.net>
util/packer: Set DEBIAN_FRONTEND=noninteractive

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Merge pull request #268 from flynn/release-system

Support custom flynn-host deb in packer builds