Remove single-version-externally-managed in setup.cfg
Remove single-version-externally-managed in setup.cfg

The single-version-externally-managed flag is no longer
a config option in pbr, it is now the default behaviour
in normal setuptools.

Change-Id: I5173d78680afca534b28cefa155636d9d695b1d9

FedericoRessi/networking-odl,dougwig/x-neutron-vpnaas,dougwig/x-neutron-fwaas,citrix-openstack-build/neutron-fwaas,schoksey/networking-odl,CiscoSystems/networking-odl,openstack/neutron-vpnaas,schoksey/networking-odl,CiscoSystems/networking-odl,openstack/neutron-fwaas,FedericoRessi/networking-odl,flavio-fernandes/networking-odl,citrix-openstack-build/neutron-vpnaas,openstack/neutron-vpnaas,gaolichuang/neutron-fwaas,openstack/networking-odl,openstack/neutron-fwaas,openstack/networking-odl,openstack/networking-odl,gaolichuang/neutron-fwaas,mestery/x-stackforge-odl,flavio-fernandes/networking-odl