Avoid dinv checking for unused certificate files

[nhinds]

The ca-key.pem, docker-client.crt, and docker-client.key files are
generated when no certificates are present in /certs, but are not used
by docker at runtime and do not need to be checked at startup.

VIC Appliance Checklist:

• Up to date with master branch
• Added tests
  • There appear to be no tests for providing TLS certificates currently
• Considered impact to upgrade
  • Providing the extra files will continue to work, and providing no files also works as documented. Only changes behaviour for people trying to provide ca.crt/docker.key/docker.crt on their own.
• Tests passing
  • I could not find tests, but manually verified with /dinv, /dinv -tls, /dinv -tlsverify with and without certificates
• Updated documentation
  • This change brings the behaviour closer to the documentation
• Impact assessment checklist
  • I think the impact assessment checklist is only for the installer, so I have not completed one

Fixes #1930

[vmwclabot]

@nhinds, VMware has approved your signed contributor license agreement.