roles/cephadm_install: fixes

[insatomcat]

cephadm_install: move distro vars into the role

The role's per-distro variables (uid/gid, logrotate path) were only
ever consumed by cephadm_install itself, so loading them from
/vars/{Distro}_cephadm_install.yml at playbook level added no value
over the standard in-role vars/ convention already used by roles like
snmp and timemaster. Move them to roles/cephadm_install/vars/{Distro}.yml
and include_vars from within the role's own tasks/main.yml, dropping
the now-unneeded include_vars step from cluster_setup_cephadm.yaml.

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cephadm_install: detect the ceph image version from local images

build_debian_iso now embeds the latest cephadm container image, so the
hardcoded cephadm_release default can diverge from the image actually
present on the nodes and break the deployment.
When cephadm_release is not set in the inventory, detect the version of
the quay.io/ceph/ceph image already present on the nodes and use it for
the whole deployment. The detection fails if a node holds several ceph
image versions or if two nodes hold different versions. When no image is
found anywhere, fall back to the default version. Setting
cephadm_release in the inventory disables the detection and pins the
version.

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cephadm_install/README.md: fix copy-paste in registryurl description

cephadm_install_registryurl was documented as "Name of the ceph release
for repo install (fallback: cephadm_release_name)", copied from the row
above. It is the image used to start the local Podman registry and has
no fallback variable.

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cephadm_install: derive the Ceph release name from the release version

"cephadm add-repo" takes a release name while the cephadm download URL
takes a version number. Both were pinned separately, so bumping
cephadm_install_release alone silently left the repo on the previous
release, mixing a binary from one Ceph version with packages from
another.
Derive the name from the major version through a lookup table, and
assert before add-repo that a name is known, so an unlisted version
fails with an explicit message instead of installing the wrong repo.
Setting cephadm_install_release_name explicitly still overrides the
derivation.