Add a way for users to specify global static VM configurations in the YAML file

I was discussing the YAML config files with @ehankland.

Currently, there are two ways to configure static VMs:
- A JSON file specified with `--static_vm_file`, which populates a global pool.
- VM group configurations within a YAML benchmark configuration.

When a benchmark needs to acquire VMs in the provision stage, it first uses the specifications from the YAML VM group's list, then pulls available static VMs from the global pool, then provisions the remainder from the configured cloud provider.

It could be useful to allow a global-level static VM list in the YAML config file, which could be used to populate the global pool. Then the separate JSON file could be deprecated, and PKB configuration would be limited to the YAML configs and the command-line flags.

Currently, the config options for a static VM in the JSON file and YAML file differ, but it would be a plus if the JSON contents could just be copied into the YAML file under the new key and continue to work.