This patch introduces the following changes.
1. Multiple external networks are created as part of rally context for dynamic workloads.
These external networks have CIDR 172.31.(2*(i-1)+1).0/23 and gateway 172.31.(2*(i-1)+1).1,
where i is the segmentation ID of the VLAN interface.
2. External networks created as part of context are used in a round robin fashion by the
iterations in dynamic workloads.
3. Swapping floating IPs between servers and trunk subports dynamic workloads are redesigned
to perform swapping within the same external network, as swapping across external networks
leads to reachability issues.
Change-Id: I130c6b524174398d66258a57e7250088992a51b8
4f0ae8d20a