MXS-2313: Expose only two rank values

The rank can now only be used to define two groups of servers: primary and
secondary servers. This limits the exposure and reduces the number of
possibilities that can arise from the use of this parameter thus making it
more predictable.

server/core/config.cc

@@ -425,7 +425,8 @@ const MXS_MODULE_PARAM config_server_params[] =
     {CN_SSL_CERT_VERIFY_DEPTH,       MXS_MODULE_PARAM_COUNT,  "9"},
     {CN_SSL_VERIFY_PEER_CERTIFICATE, MXS_MODULE_PARAM_BOOL,   "true"},
     {CN_DISK_SPACE_THRESHOLD,        MXS_MODULE_PARAM_STRING},
-    {CN_RANK,                        MXS_MODULE_PARAM_COUNT,  "2147483647"},
+    {CN_RANK,                        MXS_MODULE_PARAM_ENUM,   DEFAULT_RANK,
+     MXS_MODULE_OPT_ENUM_UNIQUE, rank_values},
     {NULL}
 };