Static VMs are re-used if multiple benchmarks are included in the same command line

If a benchmark requires more VMs than the number of static VMs provided, PKB allocates VMs from a the cloud.

But when multiple benchmarks are provided on the command line, PKB only uses the static VMs on the first few benchmarks. That is, if there 10 benchmarks each using one VM, and two static VMs are provided via the `--static_vm_file` flag, then the first two benchmarks will use a static VM and the rest will allocate one from the cloud.