Allow ramdisk-create to run without $USER set
Allow ramdisk-create to run without $USER set

It's possible this is run form an environment where $USER isn't set,
properly fallback to whoami in this case.

Change-Id: I1181f714c3c456ee264b34d282bac5c0adb67a0e

zioproto/diskimage-builder,zioproto/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder