rgw: the metavariables in frontends-related config won't be expanded

rgw: the metavariables in frontends-related config won't be expanded

When using metavariables in frontends-related config, I found that metavariables won't be expanded. Such as:
the `name` variable won't be expanded.
```
[client.rgw]
rgw frontends = civetweb port=8000 access_log_file=/path/$name.access.log
```
The reason is as follows:

In `rgw_main.cc/main()` function, program uses frontends-related config before the `expand_all_meta` function call that is called in the `global_init` function.

Signed-off-by: chnmagnus chnmagnus@qq.com