Ironic: Set DEFAULT_IMAGE_NAME to full disk image for agent
Ironic: Set DEFAULT_IMAGE_NAME to full disk image for agent

The agent_* drivers in Ironic are not yet capable of deploying partition
images. The code in DevStack was aware of that but it was looking at the
agent_ssh driver specifically.

This patch is fixing this assumption and extending that conditional for
all agent drivers.

Change-Id: I416faa9ef6fba9621cd664d5a0747b1e80e281d1

noironetworks/devstack,openstack-dev/devstack,bswartz/devstack,noironetworks/devstack,dtroyer/devstack,nawawi/openstack,Mahito/devstack,nawawi/openstack,bigswitch/devstack,group-policy/devstack,bigswitch/devstack,richliu/devstack,bswartz/devstack,samgoon/devstack,dtroyer/devstack,openstack-dev/devstack,group-policy/devstack,samgoon/devstack,richliu/devstack,Mahito/devstack