Rework riak/bucket/:bucket/key/:key/value

[shamrickus]

What does this PR (Pull Request) do?

• This PR is not related to any Issue
  The riak/bucket/:bucket/key/:key/value endpoint should be deprecated and replaced with vault/bucket/:bucket/key/:key/value with no functionality changes.

Which Traffic Control components are affected by this PR?

• Documentation
• Traffic Ops
• Traffic Vault

What is the best way to verify this PR?

Pull the code, verify the docs build with no warnings. TO tests pass without issue. Finally, 1.1 route has a deprecation notice and the 2.0 route is moved to vault.

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct OR this PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[zrhoffman]

• Deprecation warning for the old route is there
• Unit tests pass (although no unit tests exist for the files modified)
• API tests pass (although there are no API tests for this endpoint)
  *go fmt doesn't correct anything from this PR
• Docs build without any warnings from this PR

Looks good to merge.