Better image checking in update_image command
Better image checking in update_image command

Previously we were too focused on using labels to determine how to build
images. Now rely on the images_in_use list to determine if an image
should be updated and simply iterate over provider image lists in order
to update images. This fixes several issues around updating images
properly when an image may be used by multile labels.

Change-Id: I0b9f04922c90f52ced4d4a0b03e20a1b20fa7f8a

Tesora/tesora-nodepool,citrix-openstack/nodepool,citrix-openstack/nodepool,seandst/nodepool,seandst/nodepool,Tesora/tesora-nodepool