Rename reporting-sections to json-sections

... to make the role more clear - it assigns sections to summary.json
categories.
Remove oldstable and older from p.d.o summary

There is a desire in the DDPO integration to add value to the Piuparts
column by showing more detail in the failures, and to winnow out chaff
in the reporting.

The details on how to do this are not fully developed. In the
meantime, as a first, interim, step, configure p.d.o to remove summary
reporting for section tests involving oldstable and older.
Remove stable dist results from p.d.o summary

Select this commit for further de-chaffing of the JSON summary.

Remaining sections are currently:

experimental
jessie
sid
sid2experimental
sid-nodoc
testing2sid