core: Change hostalias value to list.

Site config could have multiple hostalias keys. This could be an issue
when using files in config.d, because the settings are merged with
`lists:ukeymerge` which does not support duplicate keys.

This is solved by changing hostalias syntax to the following format:
```
{hostalias, ["a.example.com", "b.example.com", "c.example.com"]}
```
doc: Remove repeated keys from docs.

The repeated configuration keys are no longer possible.
doc: touch up the docs for multiple site config files.

Fixes #651.