non-spotify specific master resolution

* Remove spotify specific srv format and domain.
* Make the srv format configurable via a HELIOS_SRV_FORMAT environment
  variable.

dnsjava uses the dns search list in /etc/resolv.conf, so users in
environments with correctly configures dns search domains will not need
to enter fully qualified helios domain names during normal use.
I've seen at other places some schemes where protocol is not part of the srv name, and there are no _'s at all, so the srv name would be something like <service>.datacenter.prod.foo.com, i.e. the SRV_FORMAT would be just `%s.%s`