Fix log formatting issue in agent base
Fix log formatting issue in agent base

This fixes a TypeError that would occur when trying
to provision an instance with local boot via IPA.

Change-Id: Ib9af83a831be5c5f344f0745e9b15d4a59ac5dac
Closes-bug: #1503008

NaohiroTamura/ironic,bacaldwell/ironic,ionutbalutoiu/ironic,NaohiroTamura/ironic,openstack/ironic,SauloAislan/ironic,ionutbalutoiu/ironic,SauloAislan/ironic,openstack/ironic,pshchelo/ironic,bacaldwell/ironic,dims/ironic,hpproliant/ironic,dims/ironic,pshchelo/ironic,devananda/ironic