Fix garbled config display in dashboard when the value is originally from environment variables.
For example, `env EMQX_STATSD__SERVER='127.0.0.1:8124' . /bin/emqx start` results in unreadable string (not '127.0.0.1:8124') displayed in Dashboard's Statsd settings page.
Related PR: [HOCON#234](https://github.com/emqx/hocon/pull/234).