Fix PXEDeploy class docstring
Fix PXEDeploy class docstring

I don't know for how long that docstring is there, but the PXE driver
is definetly not stub and have been ported long time ago.

Change-Id: I41febedf25ac9f0e771c5338460238b38f532fba

ionutbalutoiu/ironic,NaohiroTamura/ironic,redhat-openstack/ironic,SauloAislan/ironic,SauloAislan/ironic,ionutbalutoiu/ironic,rackerlabs/ironic,bacaldwell/ironic,devananda/ironic,naototty/vagrant-lxc-ironic,pshchelo/ironic,rdo-management/ironic,openstack/ironic,bacaldwell/ironic,naototty/vagrant-lxc-ironic,hpproliant/ironic,pshchelo/ironic,Tehsmash/ironic,supermari0/ironic,dims/ironic,NaohiroTamura/ironic,dims/ironic,Tan0/ironic,ramineni/myironic,naterh/ironic,openstack/ironic