live migrate conductor tasks to use nova.image.API

While the patch (I67e7c9d841e5bad82cdca8f7048eaa57f0cfbb2f) was being
reviewed, live_migrate functionality was moved to the conductor. This
live_migrate task code needed to be updated to refer to the new
nova.image.API object instead of the old nova.glance.GlanceImageService
object, which had a slightly different API.

Change-Id: I7ddb778432643f4b5371d11b675951fc678135c5
Closes-bug: #1326926