Clean up kube features, document crate features

[divergentdave]

This PR does the following:

• Clean up the janus_server self-dev-dependency as discussed in janus_cli: kube uses rustls rather than OpenSSL even in debug builds #327.
• Extends the create_clusters() test in janus_core to test connecting to the clusters.
• Fix up janus_core dev-dependencies, so that cargo test -p janus_core works. (previously, this relied on Cargo coalescing features set by janus_server's dev-dependencies when running cargo test)
• Document all of our crates' features.
• Document the cargo run --bin janus_cli --features kube-openssl suggestion in the context of the features list.

With this, I think we can close #327.