Agent driver doesn't always pxe boot before trying to clean

ironic doesn't always try to set a node to PXE boot when preparing a node for cleaning using the agent driver.

This is the result of Change-Id Ia424db63547e91e44fe18a9044b6a490e641e887 (commit 72b20fec757094031e770ee04ca9b023a5a6a44e) which changed the manage_tftp config option to manage_agent_boot, which is an overloaded option that configures whether ironic actually hosts the agent.

If manage_agent_boot is False but the agent driver is still being used, ironic won't set the node to PXE boot back into the agent before cleaning. There are cases where it's not desired that ironic host the agent image but we still want to PXE boot from another source. This was a change in default behavior from before the change; with manage_tftp = False, the agent driver would still try to PXE boot.