Extend the API with the ability to build config drives from meta_data,
network_data and user_data, where meta_data and network_data are JSON
objects, and user_data is either a JSON object, a JSON array or
raw contents as a string.
This change uses openstacksdk (which is already an indirect dependency)
for building config drives.
Change-Id: Ie1f399a4cb6d4fe5afec79341d3bccc0f81204b2
Story: #2005083
Task: #29663