Don't use ephemeral volumes for Ironic agent driver

The agent driver (unfortunately) does not support ephemeral volumes
yet. This change sets ephemeral volume size to 0, which is checked
by Tempest when testing ephemeral support.

See also: https://review.openstack.org/#/c/120814/

Change-Id: Iafac1521ff47f881934c4c6e03818486ca574cb8
Merge "Don't use ephemeral volumes for Ironic agent driver"