This change marks the iscsi deploy interface as deprecated and
stops enabling it by default.
An online data migration is provided for iscsi->direct, provided that:
1) the direct deploy is enabled,
2) image_download_source!=swift.
The CI coverage for iscsi deploy is left only on standalone jobs.
Story: #2008114
Task: #40830
Change-Id: I4a66401b24c49c705861e0745867b7fc706a7509