Add ListOfDictOfNullableString field type
Add ListOfDictOfNullableString field type

A new field type is added, it will be a list, with each item
in the list is a dict of string.

Implements: blueprint make-resource-tracker-use-objects

Change-Id: I4d3a310800afe344bc18c755658fc1dc3e792de8

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects