Expose {set,get}_boot_device in the API

A new /nodes/<uuid>/management/boot_device subresource added exposing
ways to the client to {set,get} the boot device for a node.

* To set a boot device:

 PUT {'boot_device': 'pxe' } /nodes/<uuid>/management/boot_device[?persistent=<bool>]

* To get the current boot device:

 GET /nodes/<uuid>/management/boot_device[/supported]

 The /supported flag is used to list all the boot devices that is
 supported for that node.

Implements: blueprint new-management-interface
Change-Id: I7ab93b95112efb69cf7b2daab6bbd925cb2d1e02
Merge "Expose {set,get}_boot_device in the API"