renderspec can now get a global-requirements.txt file to insert
versions into rendered spec files.
Example:
renderspec --requirements global-requirements.txt example.spec.j2
If a version is explicitly given via the py2pkg() method,
this version is used. If no version is given, renderspec tries
to find a version from the global-requirements.txt file.
Also update the documentation and for the new feature and simplify
the unittest by using ddt.
Change-Id: Ic0058f8aa9c6a1e7250e0e08f262bb3e2663d68b
5f46f91121