Previously, running neutron_ovs_cleanup on an installation with
5000 ports would time out even after setting the timeout to 3
hours. The code would do a bridge and port "by name" lookup for
every port due to being based off the ovs-vsctl implementation
where names are how everything is looked up. With this change,
the same test runs in ~1.5 mins.
This implementation adds a new OVSDB command that just looks up
the bridge, iterates over its ports, and deletes the ones that
should be deleted in a single transaction per bridge.
Change-Id: I23c81813654596d61d8d930e0bfb0f016f91bc46
(cherry picked from commit fef374131b5df33ab0e5516ba3f03c90089726dc)