Fix wrong vif driver by default

Some changes have been done in Nova to have generic drivers:
http://tinyurl.com/VifDriver

This patchs aims to change default driver.

Closes-bug #1259139
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Change-Id: I38ab21b7a3ae0f5b9ec4f08f5e30d2da58381cf1
Merge "Fix wrong vif driver by default"