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

wolverineav/networking-bigswitch,stackforge/networking-bigswitch,openstack/networking-bigswitch,wolverineav/networking-bigswitch,openstack/networking-bigswitch,stackforge/networking-bigswitch