snapshots of cloud images will share an iSCSI initiator name

After fixing LP: #1444992, smoser pointed out that there is still a gap if a cloud-image is booted and then the once-run instance is used as a template for future instances, as the iSCSI initiator name has been generated.

From IRC:

11:35 < smoser> nacc, there is 'machine-id' which is systemd's attempt at something pretty similar to what an instance-id woudl provide.
11:38 < nacc> smoser: ah so we could, in theory, store /etc/machine-id in the initiatorname.iscsi file (in a comment) and say it only applies to that config. then on startup
              in the script, check if /etc/machine-id matches what's in the iscsi initiator file and leave it if so, re-gen otherwise?
11:41 < smoser> yeah, tahts kind of what i was suggesting.
11:41 < smoser> that doesnt really fix the issue of having to clean though