[CLI] server-add should provide an option to use client certificate for authentication

Currently the only option to authenticate on the-node-to-be-added is basic auth:

--server-add-username <username>

                              The username for the server to add

--server-add-password <password>

                              The password for the server to add

Since the-node-to-be-added might be configured to use client certs for authentication, we need an option for that as well. Something like: --server-add-use-client-cert.

Note that admin doesn't need to provide the certificate/key itself. The-node-to-be-added should already be provisioned with a client certificate. The only purpose of this new option is an indication that client cert auth should be used during addition.

UPDATE:
When client certificate is selected, CLI should pass clientCertAuth=true in /addNode and /doJoinCluster calls. Username and password parameters should be skipped in this case.