dummy: Make --enable-dummy=override replace all dpifs, netdevs by dummies.

dummy: Make --enable-dummy=override replace all dpifs, netdevs by dummies.

Plain "--enable-dummy" just creates new dummy dpif and netdev classes.
This commit makes "--enable-dummy=override" go a step farther and actually
delete and replace all the existing dpif and netdev classes by copies of
the dummy class.

This is useful for testing in an environment where changing the classes in
Bridge or Interface records is challenging.

Requested-by: Andrew Lambeth <wal@nicira.com>
Tested-by: Andrew Lambeth <wal@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>