Benchmark: support configurable filesystems for each Gitaly instance

Goals:

• Support mounting XFS devices to gitaly instances
• Support mounting Btrfs devices to gitaly instances. Reference btrfs setup to tackle this.
• Rework the creation of the shared repositories disk to support the different filesystems for each gitaly.

Limitations:

• Benchmarking currently spins up a prepare VM that will clone all the git repositories defined in the config.yml to a disk and attach it to the gitaly instance. The prepare VM will only 'prepare' the disk in one filesystem. So it will not be able to spin up multiple gitaly instances with different filesystems defined. We would need to remove this prepare VM and offload its responsibilities to individual gitaly instances. Downside of this approach would be more network traffic and we lose the performance of having pre-cloned disks and just attaching to multiple gitalies that would use the same FS type. (under investigation on this approach)