Replace repo_ca_file setting with repo_certificate and repo_ca_certificate

[ErinCall]

I went ahead and removed repo_ca_file outright, rather than adding new options, mainly because I didn't want to deal with the "certificate file and contents both provided" edge-case.

For #74.

Pre-merge checklist:

• [x] Code changes have tests
• [x] Any config changes are documented:
  • If the change touches required config, there's a corresponding update to README.md
  • There's a corresponding update to docs/parameter_reference.md
  • There's a pull request to update the parameter reference in drone-plugin-index
• [x] Any large changes have been verified by running a Drone job