Only print servers that aren't destroyed

If a server were to be destroyed, it should not show up among
non-destroyed servers. Even though the servers aren't actually destroyed,
it hides unnecessary information from the user.

include/maxscale/server.h

@@ -115,6 +115,7 @@ typedef struct server
     long           persistpoolmax; /**< Maximum size of persistent connections pool */
     long           persistmaxtime; /**< Maximum number of seconds connection can live */
     int            persistmax;     /**< Maximum pool size actually achieved since startup */
+    bool           is_active;      /**< Server is active and has not been "destroyed" */
 #if defined(SS_DEBUG)
     skygw_chk_t    server_chk_tail;
 #endif