[serve] Require backend when creating endpoint

[edoakes]

Why are these changes needed?

Currently you can create and endpoint with no backends. This is a footgun for users and doesn't make conceptual sense. This change forces users to specify an existing backend when creating an endpoint to service the traffic. This also prevents users who only use a single backend from having to think about the set_traffic API.

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/latest/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failure rates at https://ray-travis-tracker.herokuapp.com/.
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested (please justify below)

[AmplabJenkins]

Can one of the admins verify this patch?

[bllchmbrs]

LGTM

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