[nova] Changes for out-of-tree drivers

I have a change proposed [1] as part of the work to add vTPM support to nova that will modify the arguments for the 'unrescue' function. As noted in the commit message, this is expected to gain a 'context' argument and lose the currently unused 'network_info' argument. If you maintain an out-of-tree driver, you will need to account for this change. Cheers, Stephen [1] https://review.opendev.org/#/c/730382/