config: add low priority env source

The usual environment configuration source is useful for parametrized
run:

```
TT_MEMTX_MEMORY=<...> tarantool --name <...> --config <...>
```

However, sometimes a user may need to set a default value, which doesn't
rewrite one that is provided in the configuration. Now, it is possible
to do using the environment variables with the `_DEFAULT` suffix.

```
TT_MEMTX_MEMORY_DEFAULT=<...> tarantool --name <...> --config <...>
```

This feature may be especially useful for wrappers that run tarantool
internally with some default paths for data directories, socket files,
pid file. We likely will use it in the `tt` tool.

Part of #8862

NO_DOC=added into https://github.com/tarantool/doc/issues/3544 manually