Simply use the scratchdisk for cinder volumes

This avoids the copying around mess and also helps with
improving performance.
Always use IPv4 for downloading the cirros image

The IPv6 address might not be reachable due to routing issues.
Remove all floating ips before running tempest

Tempest can't deal with extra floating ips and this
increases also test coverage, so..
Use sequential tempest run

Reduces memory pressure and helps actually understanding
the error messages