conf.py: _map_distro properly for '*-proposed'

There was one active section in piuparts.conf.pejacevik that didn't
map properly, squeeze2wheezy-proposed. Strip e.g. '-proposed' from the
distro name before processing.
conf.py: Add _map_distro() support for all others

Map rc-buggy and Debian<x>.<y> to the standard distribution names.
Also, re-enable the standard name tests for sid and experimental -
they are not time sensitive.