Support lack of ephemeral volumes for baremetal

The agent driver in Ironic does not yet support ephemeral volumes.
This change allows tempest to test Ironic with the agent driver
by skipping ephemeral volume checks if the ephemeral volume size
is 0.

Change-Id: I9525d1c75007555d368a2aa66ab840f41b584a30
Merge "Support lack of ephemeral volumes for baremetal"