Always add hyperv to the ML2 mechanism_drivers

Without this, networking won't work for Hyper-V instances. And it
shouldn't hurt for people not running Hyper-V as it won't do anything
in that case.
Merge pull request #44 from vuntz/fix-hyperv

Always add hyperv to the ML2 mechanism_drivers
Merge pull request #46 from crowbar/release/roxy/master

Sync with roxy