Default to the ML2 plugin in Neutron instead of OVS

In Icehouse, the OVS and LinuxBridge plugins are being deprecated in
favor of the Modular Layer 2 (ML2) plugin. This change modifies
devstack to default to ML2 when Neutron is used for networking.

Fixes bug 1220745

Change-Id: I9a4c84b04727a710219fc11f862a655309ffb99b
Merge "Default to the ML2 plugin in Neutron instead of OVS"