Makes vpn plugin configurable

Currently, it is hardcoded and not good for those who have their
own vpn plugin developed.

Change-Id: I6244040dd2abea016556cd515e06f53edbb993a0
Closes-bug: 1339977
Adds support for Hyper-V image formats

Adds support for VHD and VHDX image formats, including gz compression.

Change-Id: I9b143c9883b947397b435a671f13703f78019d23
Closes-bug: #1353726
Merge "Makes vpn plugin configurable"
Merge "Adds support for Hyper-V image formats"
Merge branch 'master' of https://github.com/openstack-dev/devstack