Don't attach to multiple Quantum networks by default

If no networking information is passed to a create request
Nova will connect the instance to all available networks.

In a Quantum configuration, where the user may have many
networks it really doesn't make sense to allow this, as the
network order is critical to most Guests and this approach
non-deterministic.

For example most guest OS's will only automatically configure
the first network which may not be externally route-able thereby
leaving the instance in an un-usable state.

To prevent the user getting into this state, the system should
reject requests with no networking information when more than
one quantum network is available, so that the user is made to
specify them in the required order.

Fixes bug 1193376

Change-Id: I950798cc3d04994b82be16e94397c117aaceecce
Merge "Don't attach to multiple Quantum networks by default"