Expose REST-API for enabling/disabling failover behavior.

It must be possible to disable the failover behaviour without having to shut down MaxScale. This is almost the same as the HA task of being able to turn MaxScale active/passive, but not quite. When MaxScale is passive, it executes no scripts, when the failover behaviour is disabled, MaxScale will not execute the failover script but will execute other scripts.

This may mean that MaxScale needs to be explicitly aware of whether a script is a failover/switchover script. Which may be desirable anyway.