Factor out ensure_starts_with_slash() as a separate util function

[convoliution]

Impact and Context

This small behavior of "ensure an endpoint path begins with a slash" is going to be used for our Docker image model support, so this PR factors it out to a reusable utility function.

Risks

This could introduce bugs, but I think there's ample test coverage to rule it out.

Testing

• Deployed the service to dev env
  • no new service
• Used functionality on dev env
  • no new backend integration behavior
• Added unit test(s)
• Added integration test(s)

pytest test_utils/test_arg_handler.py test_endpoint/test_endpoint.py::TestEndpoint::test_path_ensure_starts_with_slash

passes on my machine in both Python 2.7 and 3.7.

How to Revert

Revert this PR.