SYSV: Implement downloading and installing SYSV packages from HTTP

pkgadd has different syntax for different sources (datastream and file
system format) which makes using a single pkgtool variable difficult.
Also, SYSV packages in datastream format don't necessarily have uniform
names.  Therefore, use the existing 'simplename' attribute to specify
the datastream file name.
urllib changed a lot between python 2 and 3, so you'll need to add the appropriate magic for `urlretrieve` to `src/lib/Bcfg2/Compat.py` and then import `urlretrieve` from there.