ManagementInterface {set, get}_boot_device() to support 'persistent'

It's common for drivers to be able to set the boot device only for the
next boot (that's the default behavior for the IPMITool for example)
but it's also possible to set a boot device as persistent so that the
configuration will persist across multiple boots. This patch is making
the 'persistent' parameter to be part of the method signature of the
set_boot_device() method and also making the get_boot_device() method
to return whether the device is set as persistent or not as part of
the response.

Implements: blueprint new-management-interface
Change-Id: Ia1f9c5449612871a379377d11fb0fe91b0958026
Merge "ManagementInterface {set, get}_boot_device() to support 'persistent'"