Move install type enablement into base element

Move whether the package or source install type is enabled out of the
source-repositories element and into the base element.
source-repositories wasn't a great fit for this functionality to begin
with.

Specify the install type for an element via the
DIB_INSTALLTYPE_<install_dir_prefix> environment variable.

Additionally with this approach, any new install type can be added
in an element, without having to update source-repositories or anything
in dib.

You could just write your install type implementation under
nova-foo-install, then define DIB_INSTALLTYPE_nova=foo in your
environment, and the scripts under nova-foo-install would get run during
the image build.

Source installs (any scripts under <install_dir_prefix>-source-install)
is the default install type for all elements.

Change-Id: I9414aca360c41e030e27d3d0c0a52d9d8e13d8b1
Merge "Move install type enablement into base element"