Add ipxe_boot_menu_url as an image property

Add an ipxe_boot_menu_url image property that can override the
xenapi_ipxe_boot_menu_url configuration so that it is user
configurable.

Change-Id: Ie199f19e19856935186d3ac7ff5075b5b153db7c
Merge "Add ipxe_boot_menu_url as an image property"