rgw/auth: expose Identity::get_account()

rgw/auth: expose Identity::get_account()

now that all identities store an optional account, expose that to the
rest of rgw with get_account(). this cleans up lots of code that
otherwise has to deal with the rgw_owner variant

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 405f93305f0d630a784b2de18a5f263b37362434)