ovsdb: Fix _nexus property of OVSDBConnection
ovsdb: Nexus raises ConnectionUp events

None of the other events work yet, and this one is a hack, but at least
it (sort of) works as expected now.
ovsdb: Remove bad test code from DSL
ovsdb: Factor example into its own function

The old switch-looking example is now factored into its own launch function
that you can run with ovsdb:example.  The normal launcher now just
registers the OVSDB component and turns on back-connections by default
(which can now be overridden with --no-auto-connect).