simplified setupTLSContext()

In config.py _load_certificates() already makes sure that CERTIFICATE_DIR is
set and if TLS is enbled that KEY and CERTIFICATE are supplied, here we only
need to decide between a full 2 Way TLS context, either to start the server in
TLS mode and/or be able to do TLS client authentication, or a simple TLS
context that will only be able to verify the remote certificate in client
requests.