Extend API multivalue fields

This patch is creating a new JsonType type that accepts all the json
primitive types: bytes, unicode, float, long, int, dict, list, None
and bool.

Since the MultiType type is no longer used anywhere in the code it's
been deleted.

Closes-Bug: #1398350
Change-Id: I2d9e4f20419ca2789c2f60034c403d28a5e2b9e8