Remove okhttp dependency

We want to remove okhttp dependency from nuxeo parent pom because it conflicts with dependency of nuxeo-java-client when a project depends on these two dependencies.

Both dependencies are only used in tests, and okhttp is only used near java client calls.
Furthermore, nuxeo-java-client should be in test scope because it is not an official Nuxeo Platform dependency and client is not tied to a specific version of Nuxeo (no need to link nuxeo and client versions).