Updated openstack/openstack
Project: openstack/python-novaclient  297ebde1d38d203699547e41e00384fdad1bb798
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Explain how to delete a metadata in aggregate-set-metadata

nova aggregate-set-metadata not given value of a key means delete the
metadata, so explain this usage in help message.

Change-Id: Ie8215bd8b47ed00e3d4686edab9e3052ae84774d