Create integration test to validate that clients can successfully pull configuration from agent

[yurishkuro]

Given jaegertracing/jaeger-client-python#171, we're missing integration tests that validate that the clients can successfully communicate with the agent to retrieve configuration like sampling strategy, throttling balance, baggage restrictions, etc.

One approach we could test this is to expose an introspection API from the clients for all the dynamic configuration, and when creating the crossdock images register a server that exposes that API via some RPC API (REST or grpc). The test_driver can have a test that alters the configuration in the collectors and verifies that it is propagated to each client.