Use of a single perfkit_sc_group is undesirable

All PerfKit Benchmarker runs on OpenStack use perfkit_sc_group, rather than creating a new, unique security group rule. The security group rule is the only artifact left behind from a PerfKit Benchmarker run. I think that any artifact left behind is undesirable. Also, the use of a common security group rule for all workloads seems inflexible, should particular workloads have specific requirements that other workloads don't have.