Allow name in the DNS service watcher's ping method to configurable

https://github.com/airbnb/synapse/blob/master/lib/synapse/service_watcher/dns.rb#L18 is hard-coded to airbnb.com .

Ideally this'd be configurable, so we can set it to something internal, so we don't have to depend on airbnb.com's DNS.

Start in lib/synapse/service_watcher/dns.rb at line 18 and trace how the watcher's ping method receives its settings. Done means the DNS name used for pinging can be configured to an internal name instead of always depending on airbnb.com.

Written by the indexing model from the issue text.