threadgroup: use threading rather than greenthread
threadgroup: use threading rather than greenthread

Since threading is monkey patched in case we use eventlet, this is
change is a no-op. It buys use less usage of eventlet modules in the
end.

Change-Id: I7d39eb71b47ec00ed1c1256d94cfd4de80e3bdd5

poznyakandrey/oslo.service,eezhova/oslo.service,citrix-openstack-build/oslo.service,openstack/oslo.service