rgw: support multiple host names

Fixes: #7467
Allow configuration of multiple domain names. Still support a single
host name via the rgw_dns_name configurable but can add more names
through the region params.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
should this include the subdomain too, if it's not used for bucket lookup, or is that not a valid configuration?