Document and test proxy's soft dependencies

[GeorgianaElena]

The etcd3 and python-consul2 libraries are not installed anymore by default, when installing jupyterhub-traefik-proxy(#127). This libs are only required by the key-value store variants of the proxy, so TomlProxy shouldn't need them (checkout #125 for more context).

We should:

• Document this
• Add a test that checks that TomlProxy doesn't require these libraries and works without them (I suspect some refactoring of the testing infrastructure needs to happen before too, because the proxies are a bit "tied together" in the CI)