CHEF-5134: Warn on no SSL verification

only checking before we register the node to avoid
spamming SSL verification errors on every single connection.

because its in register it shouldn't warn in solo/apply
use cases (you still can use remote_file in those so maybe
we should also warn there?  i decided that'd be more annoying
than typically useful, YMMV...)
use local config var in #register
Check SSL configuration once per client run.

We want to ensure users see this. Also add a longer description of the
risks of the verify_none setting and a pointer to `knife ssl check` for
troubleshooting help. The warning message looks like this:

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SSL validation of HTTPS requests is disabled. HTTPS connections are still
encrypted, but chef is not able to detect forged replies or man in the middle
attacks.

To fix this issue add an entry like this to your configuration file:

```
  # Verify all HTTPS connections (recommended)
  ssl_verify_mode :verify_peer

  # OR, Verify only connections to chef-server
  verify_api_cert true
```

To check your SSL configuration, or troubleshoot errors, you can use the
`knife ssl check` command like so:

```
  knife ssl check -c /etc/chef/client.rb
```

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Add documenation for SSL warnings
Merge branch 'CHEF-5134'