ovs-brcompatd: Run ovs-vsctl instead of accessing database directly.

ovs-brcompatd: Run ovs-vsctl instead of accessing database directly.

ovs-vsctl is carefully written to avoid races in database access.  It is
much simpler to just call it than to reimplement its capabilities.

This eliminates the requirement that bridges managed by ovs-brcompatd have
no ports at ovs-brcompatd startup time.  It also eliminates races between
competing brctl and ovs-vsctl processes.