Drop CentOS 8 testing, add Ansible 2.17 testing and fix issue installing Smallstep CLI packages > v0.27.2

[trfore]

release_summary: Fix installing Smallstep CLI > 0.27.2, add testing for Ansible 2.17, and remove testing/support for CentOS 8

breaking_changes:

• Remove testing support for CentOS 8 due to EOL.

bugfixes:

• Issue [Bug]: Unable to use the collection to automatically pull the recent release, 0.27.4 #16

trivial:

• Update tox to test Ansible 2.17.
• Unpin Ansible python pkgs as upstream repos are pinning and validating 'requests' pkg.
• Validate against step-ca and step-cli 0.27.4 to 0.28.0.