Support for providing SOURCES dir and optional source tarballs

It also plugs in running spectool to automatically download
sources and patches which have URLs defined in their spec files.

Since we're downloading source tarballs from via spectool,
make --source optional.

And add option to provide a SOURCES dir for resources not
available over http for spectool.
(cherry picked from commit 25bad11)

Call it sources-dir.
(cherry picked from commit 4b46582)
Print errors from docker
Making pylint a bit happier: alignment, line wrapping and removed unused import