nuke: don't require noipmi in ctx

This is called from run.py too, which won't have ctx.noipmi.
The default of using impmi is fine for now for run.py.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
(cherry picked from commit d7fe5c0a3418355a487d65b96ad26aa181687ff8)