Use authenticated access for multi-node Citus install

[eradman]

Users may elect to use trust authentication to a local area network, but there is probably no compelling reason to allow this, even for a trial install.

• Merge worker_security section with debian and rhel installation instructions
• There should be no need to change the default rules for localhost

[jonels-msft]

Thanks for helping to improve security,. I'll review this next week.