rgw/auth: Identity::is_identity() takes one Principal

rgw/auth: Identity::is_identity() takes one Principal

take a single Principal instead flat_set<Principal>, and iterate over
calls to is_identity() instead

why?
* it simplifies the logic of each is_identity() function because they
  can use early returns to avoid visiting all of the cases
* Statement::eval_principal() no longer has to allocate a flat_set
  with a single element when the Identity is a role
* rgw::auth::Identity no longer depends on rgw::iam's choice of
  container type

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 05c15502e8cd7f8a7d279d562e2c7abadcaaeafa)