Update requirements for Xena
Update requirements for Xena

Specify a minimum of the early Xena cinderclient release (8.0.0)
and the official Xena os-brick release (5.0.0).

Change-Id: If6cda7fe30f5f1a54c6ade878f86f615c9d01568

openstack/python-brick-cinderclient-ext,openstack/python-brick-cinderclient-ext