This patch introduces a dynamic workload which creates num_vms servers with floating IPs,
and migrates num_migrate_vms servers between computes.
It also adds a parameter in browbeat-config.yaml to specify the dynamic workloads which
the user wants to run in a given iteration.
Change-Id: Ib7eff2e12c6d256ecd1f36ad30f1b19841cb1549
f55cd6c8eb