rgw/rest: iam user policy api cleanup

rgw/rest: iam user policy api cleanup

make get_params() virtual and protected. base class always validates
UserName

add common init_processing() function that calls get_params() and loads
the user by UserName. this step happens before verify_permission()

set s->err.message in several error paths

add the xmlns="https://iam.amazonaws.com/doc/2010-05-08/" part to the
responses

return ERR_LIMIT_EXCEEDED instead of ERR_INVALID_REQUEST when
RGWPutUserPolicy exceeds the policy limit

where RGW_ATTR_USER_POLICY is missing, treat it the same way we treat an
empty map of policies. this avoids separate error paths

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 93428aa6e49da8cdd602d761eaff693449bf57f2)