Switch stable ansible job to 2.8
Switch stable ansible job to 2.8

Ansible 2.6 is EOL. Run tests with 2.8.

Change-Id: I9695af073a855dd0284fc5e8161df79db09cf313

openstack/python-openstacksdk,stackforge/python-openstacksdk,stackforge/python-openstacksdk,openstack/python-openstacksdk