Windows Support

Patch for https://github.com/containernetworking/plugins/pull/85

+ Windows cni plugins are added
   (*) win-bridge (hostgw)
   (*) win-overlay (vxlan)
+ Windows netconf unit test
+ Fix appveyor config to run the test
+ Build release support for windows plugins

Address comments

From:
    - https://github.com/containernetworking/plugins/pull/85
    - 0049c64e3f

Vagrantfile

@@ -8,14 +8,11 @@ Vagrant.configure(2) do |config|
 
   config.vm.provision "shell", inline: <<-SHELL
     set -e -x -u
-
     apt-get update -y || (sleep 40 && apt-get update -y)
-    apt-get install -y git
-
+    apt-get install -y git gcc-multilib gcc-mingw-w64
     wget -qO- https://storage.googleapis.com/golang/go1.10.linux-amd64.tar.gz | tar -C /usr/local -xz
-
     echo 'export GOPATH=/go' >> /root/.bashrc
     echo 'export PATH=$PATH:/usr/local/go/bin:$GOPATH/bin' >> /root/.bashrc
     cd /go/src/github.com/containernetworking/plugins
   SHELL
-end
+end