jsonutils: simplify simple value handling
jsonutils: simplify simple value handling

There's a few code duplications because for some reason the code
handling the recursive part got some conversion code for non-recursive
value.

Let's stack non-recursive value conversion as the top, and then use
"recursive" only where needed.

Change-Id: Ic1ecc76aba5402129a936dfb4649df0806a564a4

openstack/oslo.serialization