Add {set,get}_boot_device and get_supported_boot_devices

Add set_boot_device, get_boot_device and get_supported_boot_devices
commands to the client library.

Implements: blueprint new-management-interface
Change-Id: Ib8ba6ef0da9113967f4135dea6e20284eda9d825
Merge "Add {set,get}_boot_device and get_supported_boot_devices"