[UI] No need to ask for user/password when client certs are used during node addition

When admin is adding nodes to the cluster, there should be an option for admin to use client certificates for authentication purposes. This should work for both addition scenarios:
In case when the node is added by the cluster, admin specifies the-node-to-be-added's password.
In case when the node is added by the node itself, admin specifies the cluster password.

The node that runs/hosts the UI dialog doesn't know if the remote node accepts client certificates. Only user/admin knows it, hence the idea that the dialog should provide both options, so admin can choose the right option. For example, it might look something like the following:

( ) Username: _____ Password: ________
(o) Use configured client certificate

In case when the current node (that hosts the UI) is provisioned with a client certificate, the option "Use configured client certificate" should be selected by default.

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