Cleanup images after deployment

Ironic does not require local copies of instance images to be present in
cache after deploy. This patch unlinks instance images after deployment
finished, so that they can be deleted by cache clean up procedure and it
triggers the clean up procedure.

The patch also separates the fetching of tftp and instance images as the
tftp image should be handled during the preparation of the environment,
while the latter is only needed for the actual deployment.

Change-Id: I4b975c06e60ab0f854749c8d18fd743fe770c7b0
Closes-Bug: #1231351
Merge "Cleanup images after deployment"