Magma 30-bucket rebalance tests don't report cpu_utilization_rate correctly

In the Magma 30-bucket rebalance runs, cpu_utilization_rate isn't reported correctly. We don't see this issue in Couchstore 30-bucket test and Magma single-bucket test. All runs below were running with build 7.1.0-1934. The runs were getting cpu_utilization_rate from {}:8091/pools/default.

Magma 30-bucket rebalance

Job: http://perf.jenkins.couchbase.com/job/themis-multibucket-kv-rebalance/24/

cbmonitor graphs: http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=themis_710-1934_rebalance_fbe2

Couchstore 30-bucket rebalance

Job: http://perf.jenkins.couchbase.com/job/themis-multibucket-kv-rebalance/22/

cbmonitor graphs: http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=themis_710-1934_rebalance_015e

Magma single-bucket rebalance

Job: http://perf.jenkins.couchbase.com/job/rhea-5node1/1839/

cbmonitor graphs: http://cbmonitor.sc.couchbase.com/reports/html/?snapshot=rhea_710-1934_rebalance_14ce