[nova] Changes for out-of-tree drivers

Hi, I'm working on a nova feature which adds support for rebuilding volume backed instances which modifies the arguments for 'rebuild' method. It adds a 'reimage_boot_volume' parameter which signifies if the user has requested a rebuild operation for a volume backed instance. Currently it is only implemented by the ironic driver in the in-tree drivers. If you're a maintainer of an out of tree driver then you will need to account for it. Thanks and regards Rajat Dhasmana