Sync Component Names - Phase 2 - Update Docs

Rename 3DT to "DC/OS Diagnostics".

The GUI has said "Diagnostics" in 1.8.x, but the component was still technically called 3DT or "DC/OS Distributed Diagnostics Tool".

In 1.9 the GUI should now say "DC/OS Diagnostics".

The component itself will also be renamed (https://mesosphere.atlassian.net/browse/DCOS-13066) but this may not happen in 1.9. When it does, the systemd service will be renamed "dcos-diagnostics.service" and the repo will be moved to github.com/dcos/dcos-diagnostics