A lot of packages use StringSet, but they don't use anything else from
the util package. Moving StringSet into another package will shrink
their dependency trees significantly.
The EndpointPort struct only stores one port: the port which is used
to connect to the container from outside. In the case of the Mesos
endpoint controller this is the host port. The container port is not part
of the endpoint structure at all.
A number of e2e tests need the container port information to validate correct
endpoint creation. Therefore this patch annotates the Endpoint struct with a
number of annotations mapping "<HostIP>:<HostPort>" to "<ContainerPort>". In a
follow-up commit these annotations are used to validate endpoints in a Mesos
setup.
