[Feature]: Update testing matrix to include all CLI args for step certs

[trfore]

General problem or idea

Add coverage for PR #50 & #51, and other CLI args in step_certs & step_provisioner.

[trfore]

• Basic test added with PR ci: extend test coverage for var options in step_certs and step_provisioner #53.
• Convert Step SSH validation to Testinfra
• Add test for SSH flag
• Add test for SSH certs using a token