Adds img_config_drive in nova users docs
Adds img_config_drive in nova users docs

Added img_config_drive property usuage in nova users
config drive docs

Closes-Bug: #1879066
Change-Id: I56be0b6cda51447813c0a97b01ccb48d0c898c20

openstack/nova,mahak/nova,openstack/nova,openstack/nova,mahak/nova,mahak/nova