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"
Revert "Enable tenant isolation to tempest for neutron"

This reverts commit 3d84cf2d7c323750971cf2d27f3a4eaa26cb7a9f.

This is believed to be the cause for the massive increase in
neutron failures in the gate reseting other projects. Realize
this is just a work around.

Change-Id: Id3c59f3fe9ccbb869eb3200ef7ff2659409e2253
Partial-Bug: 1224001