marathon namer: support multiple ports

[siggy]

given a marathon application with multiple ports, the marathon namer currently supports routing to the first port returned by the marathon api:

linkerd/marathon/src/main/scala/io/buoyant/marathon/v2/Api.scala

Line 80 in fc5ce1e

addr.flatMap(_.discovery).flatMap(_.ports).flatMap(_.headOption).flatMap(_.number)

consider supporting multiple ports, similar to the k8s namer.

[olix0r]

The port name could either be encoded in the name itself (as is done in kubernetes) or this could be made an explicit configuration option on the namer.

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