Add new operation new cmd command

[arbulu89]

Description

Last piece of the workbench import. Add the cmd option to run operators.
As this action is a sensitive thing, the operator subcommand is hidden.
At the end, it is more development and debug option more than a intended usage.

Run operator related commands

Usage:
  trento-agent operator [command]

Available Commands:
  list        List the available operators
  run         Run operator

Flags:
  -h, --help   help for operator

Global Flags:
      --config string           config file (default is $HOME/.trento.yaml)
      --log-level string        then minimum severity (error, warn, info, debug) of logs to output (default "info")
      --plugins-folder string   trento plugins folder (default "/usr/etc/trento/plugins/")

Use "trento-agent operator [command] --help" for more information about a command.