setup build-x86-13 and retire build-x86-12

i built a VM for a new jenkins build box called build-x86-13, in the gnt-chi cluster. i haven't hooked it into jenkins: i did everything up to assigning it the right class in Puppet, but i think it needs some special sauce to hook into jenkins.

once that is done, build-x86-12 (or -11, whatever) needs to be retired from the gnt-fsn cluster. i can do that if necessary.