TLS support in kea-shell

Once #1661 (closed) and #1662 (closed) are done, the next step is to extend kea-shell to be able to connect to TLS socket. We don’t need python 2 support anymore. It’s ok to replace the existing kea-shell code as loNg as the new code is able to do both http and https connections.

Similar to earlier tickets, we may try to reuse code from keys-secure-shell done in #1619 (closed), although the tool should be called kea-shell.