get-openstack-images: account for ext4 overhead in PV size computation

Closes #1404 (closed)

Result:

$ k get pods -n sylva-system get-openstack-images-nst6f -o yaml|grep -5 Mi
        spec:
          accessModes:
          - ReadWriteOnce
          resources:
            requests:
              storage: 23848Mi
          volumeMode: Filesystem

Tested with a custom version of get-openstack-images (doing the push even if image already in glance) with the ubuntu-jammy-plain-rke2-1-28-8:0.2.10 image:

$ k logs -n sylva-system get-openstack-images-nst6f                             
2024-06-27 15:34:30,878 INFO __main__ <module>: os_images: {'ubuntu-jammy-plain-rke2-1-28-8': {'archive-size': '2150474837', 'commit-id': 'bfe8095e7e7e7104e790adf4f06112087e5981dc', 'commit-tag': '0.2.10', 'compression': 'gz', 'filename': 'ubuntu-jammy-plain-rke2-1-28-8.raw', 'flavor': 'plain', 'hardened': 'false', 'image-format': 'raw', 'k8s-flavor': 'rke2', 'k8s-version': '1.28.8-rke2r1', 'md5': '265da0519a295930140dbebd23802e7d', 'os': 'ubuntu', 'os-release': 'jammy', 'sha256': '02f8eda3dc4e2448f75237d6b664dbfb23aa702517ba885c9a4d46abf2763cdb', 'size': '22000000000', 'uri': 'oci://registry.gitlab.com/sylva-projects/sylva-elements/diskimage-builder/ubuntu-jammy-plain-rke2-1-28-8:0.2.10'}}
2024-06-27 15:34:31,348 INFO __main__ <module>: Working on image: ubuntu-jammy-plain-rke2-1-28-8 with MD5 checksum 265da0519a295930140dbebd23802e7d
2024-06-27 15:34:31,697 WARNING __main__ image_exists_in_glance: Image with name 'ubuntu-jammy-plain-rke2-1-28-8-sylva-diskimage-builder-0.2.10' already exists.
2024-06-27 15:34:31,698 INFO __main__ <module>: image not in Glance: ubuntu-jammy-plain-rke2-1-28-8 / md5 265da0519a295930140dbebd23802e7d
2024-06-27 15:34:31,698 INFO __main__ <module>: Pulling image: ubuntu-jammy-plain-rke2-1-28-8 from artifact uri: oci://registry.gitlab.com/sylva-projects/sylva-elements/diskimage-builder/ubuntu-jammy-plain-rke2-1-28-8:0.2.10 to /tmp/tmpks11d4iy
2024-06-27 15:38:58,546 INFO oras.logger text_handler: Successfully pulled /tmp/tmpks11d4iy/sha256:9843e3d8985477ba3c5477fcd3ebad3984a83a102d91aad916c89a5b4a89d16d.
2024-06-27 15:38:58,546 INFO __main__ <module>: Unzipping artifact...
2024-06-27 15:39:06,293 INFO __main__ unzip_artifact: Extracted '/tmp/tmpks11d4iy/sha256:9843e3d8985477ba3c5477fcd3ebad3984a83a102d91aad916c89a5b4a89d16d' to '/tmp/tmpks11d4iy'.
2024-06-27 15:40:40,357 INFO __main__ unzip_artifact: Gunzipped '/tmp/tmpks11d4iy/ubuntu-jammy-plain-rke2-1-28-8.raw.gz' to '/tmp/tmpks11d4iy/ubuntu-jammy-plain-rke2-1-28-8.raw'
2024-06-27 15:40:40,357 INFO __main__ <module>: Pushing image to Glance...
2024-06-27 15:40:40,697 WARNING __main__ image_exists_in_glance: Image with name 'ubuntu-jammy-plain-rke2-1-28-8-sylva-diskimage-builder-0.2.10' already exists.
2024-06-27 15:40:40,750 INFO __main__ push_image_to_glance: Updating image properties...
2024-06-27 15:40:40,750 INFO __main__ push_image_to_glance: Image properties to update: {'sylva/archive-size': '2150474837', 'sylva/commit-id': 'bfe8095e7e7e7104e790adf4f06112087e5981dc', 'sylva/commit-tag': '0.2.10', 'sylva/compression': 'gz', 'sylva/filename': 'ubuntu-jammy-plain-rke2-1-28-8.raw', 'sylva/flavor': 'plain', 'sylva/hardened': 'false', 'sylva/image-format': 'raw', 'sylva/k8s-flavor': 'rke2', 'sylva/k8s-version': '1.28.8-rke2r1', 'sylva/md5': '265da0519a295930140dbebd23802e7d', 'sylva/os': 'ubuntu', 'sylva/os-release': 'jammy', 'sylva/sha256': '02f8eda3dc4e2448f75237d6b664dbfb23aa702517ba885c9a4d46abf2763cdb', 'sylva/size': '22000000000', 'sylva/uri': 'oci://registry.gitlab.com/sylva-projects/sylva-elements/diskimage-builder/ubuntu-jammy-plain-rke2-1-28-8:0.2.10'}
2024-06-27 15:40:40,991 INFO __main__ <module>: Image pushed to glance with image ID 309342cf-14d4-4ee8-a755-78bbe3718093
2024-06-27 15:40:40,992 INFO __main__ <module>: Cleaning up files
2024-06-27 15:40:42,534 INFO __main__ <module>: Updating configmap
2024-06-27 15:40:42,536 INFO __main__ <module>: Finished processing image: ubuntu-jammy-plain-rke2-1-28-8
2024-06-27 15:40:42,536 INFO __main__ <module>: Images UUID map:
{'ubuntu-jammy-plain-rke2-1-28-8': {'openstack_glance_uuid': '309342cf-14d4-4ee8-a755-78bbe3718093'}}
2024-06-27 15:40:42,536 INFO __main__ <module>: Pushing ConfigMap to Kubernetes...
2024-06-27 15:40:42,561 INFO __main__ create_or_update_configmap: ConfigMap updated. Name: openstack-images-uuids
2024-06-27 15:40:42,561 INFO __main__ <module>: We're done