Add option for authentication using HTTPS client certificates

[fnoorden-vs]

We use a HTTPS client certificate and key to authenticate at the API server. A login option with --cert and --key would be appreciated. Client.py would also have to be changed, like below: response = session.request( method, uri, data=data, headers=headers, auth=auth, cert=(path and filename certificate, path and filename key), verify=self._verify_ssl_certs)

[hodgesrm]

Hi @fnoorden-vs, we will look at this. Not sure about the level of effort to implement but we'll annotate the issue with a suggested implementation as soon as we have time.