Adding support for (optional) configuring of port ranges for the DEA droplets.

Currently, the DEA nodes use the ephemeral port range from the underlying OS, for picking up a port number to supply to each droplet. This range can be configured, and probably the network stack should be restarted for the change to take effect. 

This pull request provides a new configuration property for DEA nodes: port_range, which allows you to specify some range, in terms of Ruby range literal, that will be considered when choosing the port. DEA will try to pick up a random port within the range, which is free, if possible. If this fails, a fallback variant is applied, which is the default behaviour - using the ephemeral port range. This has the advantage that only the DEA must be restarted in order for the configuration change to take effect.

In a secured corporate setup, the allowed network ports are restricted and tightly controlled. Most administrators would require to be able to specify some port range, rather than having them randomly chosen.

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