util/packer: Set VirtualBox hostname to "flynn"

Signed-off-by: Lewis Marshall <lewis@lmars.net>
util/packer: Remove dhclient pre-up sleep patch

This was causing the network to not come up with errors like the
following in the boot log:

cloud-init-nonet [20.93] waiting 120 seconds network device
cloud-init-nonet [148.93] gave up waiting for network device

Signed-off-by: Lewis Marshall <lewis@lmars.net>
Merge pull request #388 from flynn/vagrant-build-fixes

Vagrant build fixes