Support installing packages by default

Adds new disk-image-create --install-type option which
can be used to enable alternate install types. The
default install type is 'source'.

The motivation is to eliminate elements like
enable-package-installs which require coupling and also
don't work with elements in multiple element repositories.

This patch does not prevent you from using the previous
DIB_INSTALLTYPE_ variables to customize the install type,
rather it just changes the default so you don't have to
set it so often when using things like packages.

Change-Id: Icee98440fc2251728f2dca30e7c4789a0fd89b93
Merge "Support installing packages by default"