Increase source-repositories support for tarballs

The existing support for tar files in source-repositories saves
only the contents within subdirectories contained in the archive.

This change makes the selection of contents within the tarball user-
definable based on the REPOREF specified (or overridden in the env).

A REPOREF of '*' (the current default, but deprecated) will select
the contents of subdirectories within the archive, while '.' will
select the entire contents of the archive.

For reference:
http://lists.openstack.org/pipermail/openstack-dev/2014-August/043197.html
http://lists.openstack.org/pipermail/openstack-dev/2014-August/043249.html

Change-Id: I45db42ce66bf1d63d6ab5730090bf458b1b37ce9
Merge "Increase source-repositories support for tarballs"