Remove default from image_ref options

This commit removes the weird default values for the image_ref option.
The IMAGE_ID defaults were probably a hold away from when they were
set in env from an early version of tempest. This sets no default and
lists them as required opts in the help field.

Change-Id: Iaeff5ff2278b0076306ed2cbfe592c06cfbbe316
Merge "Remove default from image_ref options"