Updated openstack/openstack
Project: openstack/nova  d6fb7cad8d08d93db7a4b45a026b6b81fdd8f4d4
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Remove nullable=True in models.py which is set by default

Tiny change: nullable=True is set by default, no need to use it explicitly.

Change-Id: I28883f60a0319d4834913fcabe3a3c04410175c2