MXS-1464 When pre-parsing, only check maxscale section

When pre-parsing the configuration file, the existence of environment
variables is only done for the [maxscale] section. For other sections
a nicer error message is obtained if the comlplaint is made when the
configuration file is actually loaded.

Mechanism for providing custom error message from the pre-parsing
function added.

server/core/config.cc

@@ -472,7 +472,7 @@ static int ini_handler(void *userdata, const char *section, const char *name, co
             if (!env_value)
             {
                 MXS_ERROR("The environment variable %s, used as value for parameter %s "
-                          "in section %s, does not exist.", value, name, section);
+                          "in section %s, does not exist.", value + 1, name, section);
                 return 0;
             }