Single hostalias key

Change hostalias configuration from multiple keys to single key with list value. Now hostaliases are defined in a list. This allows them to be overridden in the config.d files.

Site model no longer supports `all` queries (e.g. `m.site.all.hostalias`), because it relied on having multiple keys in the data. Use `m.site.hostalias` instead and you get a list of aliases.

Initial doc about config.d slipped into commit 1f353923f2ddd07cc7ec6c39915eab2c9e3b78a6.