Add pkg-map element.

Adds a new pkg-map element which can be used to install
packages based on an in element 'pkg-map' JSON config
file format.

As part of this change we expose DIB's IMAGE_ELEMENT variable
so that we can have access to the element names which are being
installed in a clean manner.

This change is intended to decouple elements from DIB
and allow new elements to support multiple distributions
(with various package naming schemes) without having to
constantly maintain DIB's various bin/map-packages files.

Change-Id: Ic0a061995e2ae708c95a535cba6237bff58f7d93
Set DISTRO_NAME in OS environment.d

Updates the various operating system elements so that we
set the DISTRO_NAME variable for each distro.

This env var is used by bin/pkg-map to set the default
distribution name for package name mappings.

Change-Id: Ib4b05eb7191dd50d25799a0bac51fd15c01b74cb