Named ports for mesos-dns integration

[awdrius]

Hi, currently if I try using mesos-dns with singularity deploys that utilize more than a single port (say port0 for http, port1 for clustering, etc.) mesos-dns will just lump SRV records together instead of generating SRV records per port (so something like port0.{request name}.domain. port1.{request name}.domain.).

According to https://github.com/mesosphere/mesos-dns/issues/478 mesos-dns works fine with marathon by using named ports.

By adding port naming feature to singularity it would allow mesos-dns to be used as a service discovery using DNS names like {port name}.{request name}._tcp.singularity.mesos for SRV queries.