[modules.cmd] Clean up the runas functionality
[modules.cmd] Clean up the runas functionality

    - Raise CommandExecutionError exceptions  when
      runas isn't supported or the user is invalid
    - Allow running: salt '*' cmd.run whoami "" jeff
    - Autodetermine the default cwd at _run() time
    - Don't print the su - $user -c $command in the
      log when using the runas functionality

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt