Allow None for block_device_mapping_v2.boot_index

The legacy v2 API allowed None for the boot_index [1]. It
allowed this implicitly because the API code would convert
the block_device_mapping_v2 dict from the request into a
BlockDeviceMapping object, which has a boot_index field that
is nullable (allows None).

The API reference documentation [2] also says:

"To disable a device from booting, set the boot index
to a negative value or use the default boot index value,
which is None."

It appears that with the move to v2.1 and request schema
validation, the boot_index schema was erroneously set to
not allow None for a value, which is not backward compatible
with the v2 API behavior.

This change fixes the schema to allow boot_index=None again
and adds a test to show it working.

This should not require a microversion bump since it's fixing
a regression in the v2.1 API which worked in the v2 API and
is already handled throughout Nova's block device code.

Closes-Bug: #1662699

[1] https://github.com/openstack/nova/blob/13.0.0/nova/compute/api.py#L1268
[2] http://developer.openstack.org/api-ref/compute/#create-server

Change-Id: Ice78a0982bcce491f0c9690903ed2c6b6aaab1be

nova/block_device.py

@@ -197,7 +197,13 @@ class BlockDeviceDict(dict):
                 api_dict[source_type + '_id'] = device_uuid
             if source_type == 'image' and destination_type == 'local':
                 try:
-                    boot_index = int(api_dict.get('boot_index', -1))
+                    # NOTE(mriedem): boot_index can be None so we need to
+                    # account for that to avoid a TypeError.
+                    boot_index = api_dict.get('boot_index', -1)
+                    if boot_index is None:
+                        # boot_index=None is equivalent to -1.
+                        boot_index = -1
+                    boot_index = int(boot_index)
                 except ValueError:
                     raise exception.InvalidBDMFormat(
                         details=_("Boot index is invalid."))