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.
Also you might want to extract the string as it's own constant, like `SPOTIFY_SRV_FORMAT` since probably no one else would understand why this was the default.  Arguably, there should be no default, or it should be something fairly generic, like `%s.%s`