Agent: allow for known-credentials in addition to cbmultimanager auth

In CMOS-276 we made the cluster monitor transmit credentials to the agent, which is useful in on-premise scenarios when it doesn't know the credentials to use. However, when running under Operator management, it can be given the credentials to use, thus obviating the need for the process.

Re-instate the ability to use known credentials, falling back to activation if none are supplied.