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_runtime.cc

@@ -621,9 +621,11 @@ bool runtime_alter_server(Server* server, const char* key, const char* value)
     }
     if (strcmp(key, CN_RANK) == 0)
     {
-        if (auto i = get_positive_int(value))
+        auto v = config_enum_to_value(value, rank_values);
+
+        if (v != MXS_UNKNOWN_ENUM_VALUE)
         {
-            server->set_rank(i);
+            server->set_rank(v);
         }
         else
         {