Add ironic instance rebuild test

This adds verification of rebuilding an instance and preservation
of the ephemeral partition. A file is created on the ephemeral
partition before the rebuild, then after the rebuild, we verify
that it still exists. We also verify the expected ephemeral
partition mount point and size, which is an ironic-specific
test criteria.

Change-Id: I6a00f1835c359bfd639bf19ba2c4616638695359
Merge "Add ironic instance rebuild test"