rgw/iam: role apis override init_processing/verify_permission

rgw/iam: role apis override init_processing/verify_permission

replace get_params() with init_processing() override which runs before
verify_permission(). use this to validate request parameters and load
the existing role if necessary. simplify verify_permission() by
forwarding to RGWRESTOp::verify_permission() which calls check_caps()

simplify inheritence hierarchy by taking cap perm and iam action as
constructor arguments

Signed-off-by: Casey Bodley <cbodley@redhat.com>