Ensure that a VirtualEthernetCardNetworkBackingInfo is created when attaching to an Standard Switch Network.

This solves the follow execption, that I think is caused when the template
was created on a different kind of network.

Exception:

	/home/keymon/.chefdk/gem/ruby/2.1.0/gems/knife-vsphere-0.9.9/lib/chef/knife/vsphere_vm_clone.rb:337:in `rescue in generate_clone_spec': undefined method `deviceName=' for #<RbVmomi::VIM::VirtualEthernetCardDistributedVirtualPortBackingInfo:0x00000004a015f0> (NoMethodError)
		from /home/keymon/.chefdk/gem/ruby/2.1.0/gems/knife-vsphere-0.9.9/lib/chef/knife/vsphere_vm_clone.rb:332:in `generate_clone_spec'
		from /home/keymon/.chefdk/gem/ruby/2.1.0/gems/knife-vsphere-0.9.9/lib/chef/knife/vsphere_vm_clone.rb:231:in `run'
		from /opt/chefdk/embedded/apps/chef/lib/chef/knife.rb:493:in `run_with_pretty_exceptions'
		from /opt/chefdk/embedded/apps/chef/lib/chef/knife.rb:174:in `run'
		from /opt/chefdk/embedded/apps/chef/lib/chef/application/knife.rb:139:in `run'
		from /opt/chefdk/embedded/apps/chef/bin/knife:25:in `<top (required)>'
		from /usr/bin/knife:34:in `load'
		from /usr/bin/knife:34:in `<main>'