- :param str url: a string that how to connect to the VCA (if needed, IP and port
- can be obtained from there)
- :param str username: the username to authenticate with VCA
- :param dict vca_config: Additional parameters for the specific VCA. For example,
- for juju it will contain:
- secret: The password to authenticate with
- public_key: The contents of the juju public SSH key
- ca_cert str: The CA certificate used to authenticate