Allow block devices without device_name

This patch removes the check in the API that prevented having block
devices without 'device_name' and makes the compute manager capable of
guessing those names.

Before creating the instance, the manager will try to guess a valid name
using the method 'default_device_names_for_instance',  which should be
implemented by the drivers, or defer to the generic function from the
utils module 'default_device_names_for_instance' which uses the already
existing 'get_device_name_for_instance' function.

It also takes care of assigning the correct device name to the root
device (boot_index=0) to make it agree with the field `root_device_name'
from the instance. If none were supplied - it will try to guess it by
calling driver 'default_root_device_name' or defer to the generic
'get_device_name_for_instance'. if driver does not provide it.

What is worth noting also is that, should the drivers override the
two methods mentioned above, the code should expect to be passed the
standard block device mapping format, and not the one used in other
driver methods.

DocImpact

Part of blueprint: improve-block-device-handling

Co-authored-by: Xavier Queralt <xqueralt@redhat.com>

Change-Id: I84541f8ff6e1b5978734e5def69946d014c66fdf
Merge "Allow block devices without device_name"