hostname can set even if the 'hostname' is key

This pull-requst is variable name change `hostname` to `myhostname`.
If there is config key `hostname`, check placeholders(`${hostname}, %{hostname} or __HOSTNAME__`),
and set host name in value.


### before

```
<match pattern>
  type record_modifier
  tag foo.filtered

  gen_host ${hostname}
  foo bar
</match>
```

### after

```
<match pattern>
  type record_modifier
  tag foo.filtered

  hostname ${hostname} 
  foo bar
</match>
```