Rename imagebackend arguments.
Rename imagebackend arguments.

Rename function arguments to something which more clearly indicates
their purpose. No functional changes.

Change-Id: Iedbc5477ac54527b60c520fd7c774e608adb07c0

openstack/ironic-lib,supermari0/ironic,SauloAislan/ironic,ramineni/myironic,pshchelo/ironic,emonty/ironic,ionutbalutoiu/ironic,dims/ironic,NaohiroTamura/ironic,rackerlabs/ironic,citrix-openstack-build/ironic,JioCloud/ironic,debayanray/ironic_backup,naototty/vagrant-lxc-ironic,Tan0/ironic,naototty/vagrant-lxc-ironic,openstack/ironic,citrix-openstack-build/ironic-lib,froyobin/ironic,Tehsmash/ironic,SauloAislan/ironic,faizan-barmawer/elytics,faizan-barmawer/ironic-lib,citrix-openstack-build/ironic,varunarya10/ironic,bacaldwell/ironic,pshchelo/ironic,dims/ironic,redhat-openstack/ironic,naterh/ironic,rdo-management/ironic,openstack/ironic,hpproliant/ironic,bacaldwell/ironic,devananda/ironic,ionutbalutoiu/ironic,NaohiroTamura/ironic-lib,faizan-barmawer/openstack_ironic,NaohiroTamura/ironic