MXS-2773: Make host blocking an optional feature

In cases where servers are known to be down on startup, this feature does
more harm than good. Disabling it in these cases would be preferable but
due to how the parameter is used, it is not possible.

server/core/config_runtime.cc

@@ -928,8 +928,9 @@ bool runtime_alter_maxscale(const char* name, const char* value)
     }
     else if (key == CN_MAX_AUTH_ERRORS_UNTIL_BLOCK)
     {
-        if (int intval = get_positive_int(value))
+        if (is_valid_integer(value))
         {
+            int intval = atoi(value);
             MXS_NOTICE("Updated '%s' from %d to %d",
                        CN_MAX_AUTH_ERRORS_UNTIL_BLOCK,
                        cnf.max_auth_errors_until_block,