Fix single-version-externally-mananged typo in setup.cfg

Corrects the typo in setup.cfg to use single-version-externally-managed
for pbr.

Fixes bug 1192987

Change-Id: I039b51897ec180148e5095349e4567759f2711a5
Merge "Fix single-version-externally-mananged typo in setup.cfg"