* Extend 'backend.DiagnoseRPC' to stat 'create_untrusted', which
  indicates the OS is untrusted.
* Extend queries to include the 'trusted' field, which is calculated
  from the whether the OS contains a 'create_untrusted' script.
* Fix typo in docstring.
* Extend client to show OS trusted field.
* Extend cluster verify, which performs OS related checks, to account
  for trusted/untrusted scripts.

Signed-off-by: Jose A. Lopes <jabolopes@google.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>