Add elements to user image via TEST_IMAGE_DIB_EXTRA_ARGS

There are some elements that need to be included in the user image,
like apt-source and pypi, to be able to build it in a network restricted
environment.

Change-Id: I90d37a641c5e6ce9b98bf2e8a6cde05ef39f8cc3
Merge "Add elements to user image via TEST_IMAGE_DIB_EXTRA_ARGS"