etcd: refactor TLS configuration

Actual files should never have to be used for unit tests. Refactor etcd.TLSClientConfig to take byte slices instead.
`(ca, cert, key []byte, err error)` is simpler imo

also, I don't think there's any great need to expose both functions outside the package, `TLSClientConfig` can just call `readTLSConfigFiles` internally