get is used by __getitem__, but also by other callers as it is public
method. So deserialize values if needed on each access by get, and
remove now needless deserialization in __getitem__

As Options.get is used internally, add Options._get without
deserialisation and use it in places, which expect clean result, and
adapt Options.get to be simple wrapper with deserialization without
using decoration with functools.

/reviewed-on nexedi/slapos.buildout!10