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
Merge "Debian: Support additional debootstrap arguments"