configdrive can contain a vendor_data2.json file containing key/value
pairs injected by nova's vendordata mechanism[1].
This change lets Ironic accept a vendor_data key when configdrive is
provided as json, allowing parity with nova.
This change requires an openstacksdk release 0.37.0
[1] https://www.madebymikal.com/nova-vendordata-deployment-an-excessively-detailed-guide/
Change-Id: Id990b970619a113c5d5ead47fb550870d91b5e04
Task: 36756
Story: 2006597
Blueprint: nova-less-deploy