Add a commented-out option to allow a private network for PL/SQL debu…

[CBatemanOracle]

…gging.

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[PaulNeumann]

I like this idea, but I have a couple of comments.

• If this is implemented, I think it should be done for all of the single instance database projects, not just 19.3.0.
• I think it should be configured using environment variables (and, optionally, the vagrant-env plugin), to be consistent with the other configurable parameters, and to make it unnecessary to modify the Vagrantfile.
• I don't think the 192.168 address range will work for the libvirt provider.
• I think the README.md file should be updated to include this enhancement.