Fix ironic-python-agent-builder to 2.8.0
Fix ironic-python-agent-builder to 2.8.0

Ironic python agent builder 2.8.0 is appropriate for Wallaby releases
of OpenStack Ironic as it still supports the iscsi deployment
interface *and* contains latest fixes.

This replaces 3.x which *was* being pulled in and 2.7.x
which was also being pulled in based upon train specific
fixes.

Change-Id: I103e1933a05678efa78076e8c4f7d6d1de8b51fa

redhat-openstack/rdoinfo