Introduce a TMaybe combinator

We have many cases in the code where we write TOr(TNone, a), so let's
introduce a combinator that simplifies this case.

Beside replacing the above with TMaybe(a), I did a few other parameter
fixes:

- noop change TOr(TNone, TDict) to TMaybeDict
- noop change TOr(TNone, TNonEmptyString) to TMaybeString
- OpClusterSetParams, change master netmask from any integer to a
  non-negative integer

(I can move the last one to a separate patch if desired)

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>