Let tools/generate_units_documentation.py recognize Vault version

Vault version is determined by .spec.image of the Vault resource defined in kustomize-units/vault/base/vault.yaml (today hashicorp/vault:1.13.13), but tools/generate_units_documentation.py automatically recognised something else (1.19.0 from kustomize-units/vault/base/kustomization.yaml:10:- https://raw.githubusercontent.com/banzaicloud/bank-vaults/1.19.0/operator/deploy/rbac.yaml).