This helps parallelize the numerous Zookeeper calls a little bit, at
least within the bounds of the GIL, to improve performance when getting
a large list of VMs. The max_workers value is capped at 32 to avoid
causing too many threads during concurrent executions, but still
provides a noticeable speedup (on the order of 0.2-0.4 seconds with 75
VMs, scaling up further as counts grow).
Joshua M. Boniface
58789f1db4