20797: Streamline setup of Python build variables

20797: Streamline setup of Python build variables

Instead of setting Debian defaults and overriding them,

1. set global defaults
2. set target-specific variables
3. set deb/rpm-specific defaults as needed

I hope this makes clearer what's actually a target-specific variable
vs. a regular difference between Debian and Red Hat, and so makes it
easier to add targets in the future.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>