Introduce the concept of active/passive to MaxScale

In active mode, MaxScale behaves in the normal manner.

In passive mode MaxScale

• when started, will create all services, listener sockets, REST-API endpoint, etc. just like an active MaxScale will do,
• will expect not to receive traffic (although it will not cause errors if it does),
• will monitor the servers, and
• will not launch external programs on the basis of state changes in the servers.

Whether or not MaxScale should start in active or passive mode should be configurable in the configuration file and also possible to indicate with a command line flag.