Node setup doc: install NTP package/service by default

[abitmore]

Can we include installing NTP package/service to the default witness node setup guide, although it's not a dependency, it's best practice to have it for block producing nodes as well other nodes.

[faddat]

Great idea!

I've forgotten NTP a few times only to have "strange things" come up on the logs :)

[sneak]

We're going to stick specifically to steem-the-codebase-related things in the repo. Other system setup scripts belong in separate deployment configurations/repos.

Also, most providers and OSes ship with NTP preconfigured on nodes or VM-hosting hardware.

There are tons of system and system security configuration best practices we could include, but they're out of scope. This repo is just for steem-the-program.