GraphQL-80: rename canonical_url to relative_url and fix covered test

[pfantini]

Description

1. Rename canonical_url to relative_url.
2. Fix functional test.

Manual testing scenarios

1. Run the following request in GraphiQL:

{
urlResolver(url:"/test-product.html") {
   id
   relative_url
   type
 }
}

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)