Refactor config.py as preparation for rpm support.

Refactor config.py as preparation for rpm support.

Split GbpOptionParser in to a common base class and a Debian
specific subclass, as a preparation for upcoming RPM support.

This allows having different default values (and help texts) for common
options between different package types, and, clearer separation of
package type specific options.

Signed-off-by: Guido Günther <agx@sigxcpu.org>