Correctly setup ML2 mechanism_drivers

The ML2 code in devstack was not correctly configuring the mechanism_drivers
when asked to do so. This corrects the typo in the variable assignment, and
also actually sets these in the plugin configuration file.

Fixes bug 1208557

Change-Id: I3746ca099f45d44dcf1cc2ca1c3726745b8e8a1d
Merge "Correctly setup ML2 mechanism_drivers"