[ansible/platform] Make service user shells configurable

[omribz156]

PR Checklist

• Title of the PR starts with installer/product name (e.g. [ansible/artifactory])
• CHANGELOG.md updated
• Variables and other changes are documented in the README.md

What this PR does / why we need it:

Adds service-user shell variables for the Ansible Platform roles:

• artifactory_user_shell
• xray_user_shell
• distribution_user_shell

The defaults remain /bin/bash, so existing behavior is unchanged. Operators who need non-interactive service accounts can now set the relevant variable to /sbin/nologin, /usr/sbin/nologin, or another site-approved shell without patching the role tasks.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #488

Verification:

• git diff --check
• Parsed touched defaults/task YAML files with PyYAML.
• rg "user_shell|service user shell|shell:" -n Ansible\ansible_collections\jfrog\platform\roles\artifactory Ansible\ansible_collections\jfrog\platform\roles\xray Ansible\ansible_collections\jfrog\platform\roles\distribution Ansible\ansible_collections\jfrog\platform\CHANGELOG.md

Not run:

• ansible / ansible-lint, because they are not installed in this Windows workspace.

Special notes for your reviewer:

I kept the default shell unchanged to avoid a behavior change for existing installations; this only exposes the hardening knob.

This was implemented with Codex assistance, with the patch kept focused and manually reviewed.