ovs-vsctl: Fix br-set-external-id.

ovs-vsctl: Fix br-set-external-id.

The wrong prerequisite function was being called for set-br-external-id, so
it didn't set up to monitor the right columns, so an assertion failed.
This fixes the problem.

Also, change the test for external IDs so that it will find the regression.
(Running all of the ovs-vsctl commands together caused the column to be
monitored due to another command's intervention.)

Reported-by: Mikael Doverhag <mdoverhag@nicira.com>
Bug #4095.