[tests] Improve devstack/post playbook efficiency
[tests] Improve devstack/post playbook efficiency

By adjusting the syntax to the newer format, the fetch-subunit
role will be skipped entirely, rather than run with all its
tasks skipped. This gives us results sooner and burns less
electricity, making the world a better place. :)

Change-Id: I6ae5a63051d512eecb2a603af9d74ec139eb0877

stackforge/python-openstacksdk,stackforge/python-openstacksdk