We don't need two roles for testr and stestr. The roles have logic to
check to see if .testrepository or .stestr dirs exist, and the logic
after the subunit extraction is the same. Combine them into a single
role which should do the right thing.
Also, document it.
Change-Id: I833320cf9a932d8e119645eb798ce0c93d854321
ebdcf57fb0