This introduces airshipctl integration with image-builder [0], which
replaces the existing isogen tool for ephemeral ISO generation.
The airshipctl isogen executor has been updated for building ephemeral
ISOs using the image-builder container. The ability for user-declared
filenames for cloud-init user data and network data was removed, since
the user's only interest is in supplying the relevant overrides, not in
transparent naming coordination with the image-builder container. A new
object is added to the document package to identify the document kind,
label, and key to retrieve data from since this is pattern we will
reuse elsewhere.
Progress flag removed as requsted. Progress is reported directly by the
image-builder container.
Isogen debug flag removed in favor of using log.DebugEnabled()
[0] https://review.opendev.org/#/c/730777/
Depends-On: https://review.opendev.org/c/airship/images/+/730777/
Change-Id: I545004feaf2116f8ffb29faf6f7f7f5fcfe24fff
2daacf5f2a