fix sys.cpu.utilization metrics for titus isolate v2

[copperlight]

The new 7th generation hardware sets a cfs_quota of max, which is then read as a 0 in the number vector. This was breaking utilization calculations.

• Refactor code to remove all remaining traces of cgroup selection, since all Titus hosts are now on cgroupv2.
• Remove extra ifdef macro for TITUS_SYSTEM_SERVICE that had no effect.
• The TITUS_NUM_CPU environment variable is expected to always be present, and should always be used to determine the number of CPUs allocated.
• The number of CPUs can be used to determine the value of cfs_quota, which can then be used to calculate available CPU time.
• Refactor metrics collection for cpu time and cpu utilization, to reduce the number of duplicate calls and simplify the processing.
• Shuffle the order of nvml library initialization, so that we can use the standard logger to report status.
• Refactor the setup-venv.sh script, to make it easier to bootstrap new dev environments on bionic hosts.