Handle multiple DNS forwarders correctly

Oslo.config expects list optionts to be separated by
comma, not space (see
http://docs.openstack.org/developer/oslo.config/types.html)

(cherry picked from commit ef4b976faaf3a95797356ae2e8527d0c94f9a4fe)
Merge pull request #111 from dirkmueller/release/roxy/master

Handle multiple DNS forwarders correctly