[opentracing] Fixing context provider imports for scope manager

[Maximilien-R]

The goal is to import properly all context_providers to fix the import error when initializing a ddtrace.opentracer.Tracer object with a custom scope manager (as explained here).

[brettlangdon]

Ha! You beat me to a PR, I basically had the exact same diff

[Kyle-Verhoog]

Nice 😄. We can probably also add a regression test for this since we should cover when custom scope managers are used. But I'm fine leaving that for a separate PR too.

[Maximilien-R]

@Kyle-Verhoog The fact is that the get_context_provider_for_scope_manager function already have unit tests (both Gevent & Asyncio). The tests don't fail certainly because before their execution there is an "explicit" import of their context_provider which put them in the python import cache.

[brettlangdon]

@Maximilien-R thank you for this contribution, we just released version 0.19.0 which includes this fix.