Debian: Support additional debootstrap arguments

Add support for supplying a custom keyring and debootstrap script
using two new environment variables:

* DIB_DEBIAN_KEYRING - Path to a gpg keyring.
* DIB_DEBIAN_DEBOOTSTRAP_SCRIPT - Path to a alternative debootstrap
  script.

Change-Id: I42e8832b435b8a77b0681b5bc9d9332bbe860cd5
Add tar as an output type

Add support for d-i-b to create tarballs, the primary consumer for
tarball images would be linux containers.

Change-Id: I27d67401f3e4415226a4a51e1dde46f739c0220a
Merge "Debian: Support additional debootstrap arguments"
Merge "Add tar as an output type"