[Vagrant][HyperV] Customization

New feature for vagrant-hyperv for including networking customization during pre-boot. With this feature one can customize which virtual switch the VM can take from the Vagrantfile.

Example usage
 ```ruby
config.vm.provider "hyperv" do |hv, override|
   hv.customize  ["virtual_switch", { type: "External", name: "External Switch", :adapter => "Ethernet" }]
end
 ```