Eliminate conflicts between fixed and auto-allocated network ports

Today, when DCP allocates a random port for Service proxy, or Executable that needs a port, it will look at currently occupied ports on the machine. However, it will NOT look at ports that are pre-defined as part of Service spec or endpoint producer annotations on Containers and Executables. This might lead to choosing a (random) port that will conflict with a port that was pre-defined in the DCP model.

We should try harder to eliminate these kinds of conflicts.