added initial code to use authentication config for nessus

[jeremychoi]

added initial code to handle the existing 'authentication' config options for Nessus.

• No authentication will affect nothing
• invalid value for authentication will result in RuntimeError
  • for now, all authentication type except for 'http_header' will result in RuntimeError with an error log, as authentication methods are yet to be implemented at a later time
  • 'http_header' type for authentication will result in RuntimeError with a different specific error log as Nessus doesn't support it.