Refactor: scheduler: always preserve local node name

set_working_set_defaults() zeroes out the scheduler object, preserving only a
few specific members. Previously, unpack_cib() would preserve localhost.
However, the local node name never changes during the lifetime of a scheduler
object, and set_working_set_defaults() can be called via other paths (most
importantly cleanup_calculations()), so preserve localhost in
set_working_set_defaults() instead.