F #46: Improve infra role + Add persistent mounts

[sk4zuzu]

• Pre-run helpers python3 and cache in the infra playbook (fix).
• Do NOT use pip, use system packages instead.
• Use genisoimage instead of pycdlib.
• Use the 'pc' machine type (fix).
• Add a way to define passthrough filesystems (like virtiofs or 9p).
• Add the helper/fstab role to manage persistent mounts.