Fix list_migration response schema for None values

list migration response schema in Tempest does not allow
many of expected nullable attribute as None.

For example dest_host, source_node, dest_node can be None
for "evacuation", "resize" migration type. or in other migration
type Destination and source are mostly driven by "source_compute"
and "dest_compute" .

There is failure for that in mulitnode job -
http://logs.openstack.org/80/202380/2/check/gate-tempest-dsvm-multinode-full/623ac01/console.html

This commit fix list_migration schema as per Nova return response.

Change-Id: I5e81dadd5dee4ecd85f309c34dae0d3ba0733a0d
Closes-Bug: 1475454
1 file changed