DevStack: Make $IRONIC_IMAGE_NAME less dependent of the name in DevStack
DevStack: Make $IRONIC_IMAGE_NAME less dependent of the name in DevStack

This patch is making the $IRONIC_IMAGE_NAME less dependent on the full
name of the image in DevStack. We will need different images for the
UEFI work and this would break it so, let's make it a bit more smart.

Change-Id: Iac8668e9470bc151c10ea145d4394ba7ff7041d8

NaohiroTamura/ironic,openstack/ironic,SauloAislan/ironic,NaohiroTamura/ironic,SauloAislan/ironic,openstack/ironic,pshchelo/ironic,pshchelo/ironic