Enable multi-threaded nova-conductor

Just like I09f4c6f57e71982b8c7fc92645b3ebec12ff1348, enable
multi-threaded nova-conductor.  This feature was merged into nova in
I8698997d211d7617ee14a1c6113056a694d70620.

Change-Id: Id7042284e81bd64092a400d24a3170ce07beb08c
Merge "Enable multi-threaded nova-conductor"
Merge branch 'master' of github.com:openstack-dev/devstack