dib environment variables

DIB_DEV_USER_USERNAME="vagrant"
DIB_DEV_USER_PASSWORD="****************"
DIB_DEV_USER_PWDLESS_SUDO=true
DIB_DEV_USER_AUTHORIZED_KEYS="/home/vagrant/.ssh/authorized_keys"
DIB_DEV_USER_SHELL="/bin/bash"
DIB_DHCP_TIMEOUT=10
DIB_BLOCK_DEVICE=mbr
DIB_DISTRIBUTION_MIRROR=http://fr.archive.ubuntu.com/ubuntu