packer: use latest VirtualBox Guest Additions ISO

Using the default local Virtualbox Guest Additions ISO will result
in different builds depending on the system being used.
My Debian jessie system with Virtualbox 5.1.8-dfsg-6~bpo8+2
from jessie-backports/contrib is actually too old to build
stretch and newer as the kernel module can't be compiled.
So instead use a specific VBoxGuestAdditions ISO, currently
being the one recommended by upstream on https://www.virtualbox.org/

While at it:

* increase VM memory size from 512MB to 1GB during build, otherwise
  we can't even use a temporary apt cache directory and will fail with OOM

* check for build errors in virtualbox logs

* /var/log/VBoxGuestAdditions.log doesn't necessarily exist,
  so instead display the last 10 lines of the according log files
  that actually exist