The 'ConfigDrive' DataSource supports the OpenStack configdrive disk.
See doc/source/api_ext/ext_config_drive.rst in the nova source code for
more information on config drive.

The following criteria are required to be identified by
DataSourceConfigDrive as a config drive:
 * must be formated with vfat filesystem
 * must be a un-partitioned block device (/dev/vdb, not /dev/vdb1)
 * must contain one of the following files:
    * etc/network/interfaces
    * root/.ssh/authorized_keys
    * meta.js

By default, cloud-init does not consider this source to be a full-fledged
datasource.  Instead, the default behavior is to assume it is really only
present to provide networking information.  Cloud-init will copy off the
network information, apply it to the system, and then continue on.  The
"full" datasource would then be found in the EC2 metadata service.

== Content of config-drive ==
  * etc/network/interfaces
    This file is laid down by nova in order to pass static networking
    information to the guest.  Cloud-init will copy it off of the config-drive
    and into /etc/network/interfaces as soon as it can, and then attempt to
    bring up all network interfaces.

  * root/.ssh/authorized_keys
    This file is laid down by nova, and contains the keys that were
    provided to it on instance creation (nova-boot --key ....)

    Cloud-init will copy those keys and put them into the configured user
    ('ubuntu') .ssh/authorized_keys.

  * meta.js
    meta.js is populated on the config-drive in response to the user passing
    "meta flags" (nova boot --meta key=value ...).  It is expected to be json
    formated.

== Configuration ==
Cloud-init's behavior can be modified by keys found in the meta.js file in
the following ways:
 * dsmode:  
   values: local, net, pass
   default: pass

   This is what indicates if configdrive is a final data source or not.
   By default it is 'pass', meaning this datasource should not be read.
   Set it to 'local' or 'net' to stop cloud-init from continuing on to
   search for other data sources after network config.

   The difference between 'local' and 'net' is that local will not require
   networking to be up before user-data actions (or boothooks) are run.
   
 * instance-id:
   default: iid-dsconfigdrive
   This is utilized as the metadata's instance-id.  It should generally
   be unique, as it is what is used to determine "is this a new instance".

 * public-keys:
   default: None
   if present, these keys will be used as the public keys for the
   instance.  This value overrides the content in authorized_keys.
   Note: it is likely preferable to provide keys via user-data

 * user-data:
   default: None
   This provides cloud-init user-data.  See other documentation for what
   all can be present here.

== Example ==
Here is an example using the nova client (python-novaclien)

Assuming the following variables set up:
 * img_id : set to the nova image id (uuid from image-list)
 * flav_id : set to numeric flavor_id (nova flavor-list)
 * keyname : set to name of key for this instance (nova keypair-list)

$ cat my-user-data 
#!/bin/sh
echo ==== USER_DATA FROM EC2 MD ==== | tee /ud.log

$ ud_value=$(sed 's,EC2 MD,META KEY,')

## Now, 'ud_value' has same content of my-user-data file, but
## with the string "USER_DATA FROM META KEY"

## launch an instance with dsmode=pass
## This will really not use the configdrive for anything as the mode
## for the datasource is 'pass', meaning it will still expect some
## other data source (DataSourceEc2).

$ nova boot --image=$img_id --config-drive=1 --flavor=$flav_id \
   --key_name=$keyname \
   --user_data=my-user-data \
   "--meta=instance-id=iid-001 \
   "--meta=user-data=${ud_keyval}" \
   "--meta=dsmode=pass" cfgdrive-dsmode-pass

$ euca-get-console-output i-0000001 | grep USER_DATA
echo ==== USER_DATA FROM EC2 MD ==== | tee /ud.log

## Now, launch an instance with dsmode=local
## This time, the only metadata and userdata available to cloud-init
## are on the config-drive
$ nova boot --image=$img_id --config-drive=1 --flavor=$flav_id \
   --key_name=$keyname \
   --user_data=my-user-data \
   "--meta=instance-id=iid-001 \
   "--meta=user-data=${ud_keyval}" \
   "--meta=dsmode=local" cfgdrive-dsmode-local

$ euca-get-console-output i-0000002 | grep USER_DATA
echo ==== USER_DATA FROM META KEY ==== | tee /ud.log

--
[1] https://github.com/openstack/nova/blob/master/doc/source/api_ext/ext_config_drive.rst for more if