Issue #2870 - Always empty PXEClient on native_ms.rb server

Often DHCP scopes have PXEClient set by default. This interferes with
the way Foreman does pxe/tftp. With this patch the default is to set
PXEClient to empty string when using native_ms.
Merge branch 'pxeclient_emptied' of github.com:tomas-edwardsson/smart-proxy into pxeclient_emptied