JoinMirror indicates whether the system processes its mirror configuration at startup.
1 (true) - The mirror configuration is processed and the system is considered a mirror member according to its configuration.
0 (false) - The mirror configuration is ignored and the system is not initialized as a mirror member. Use this if there is a problem in the configuration which prevents the system from starting or if the member must be reconfigured before joining the mirror. For example, if a system was the primary but no longer is, prevents the system from joining the mirror when it restarts and attempting to become the primary again, which could result in dual primaries.