When performing a resize, we'll want to (by default) select
target hosts from the source cell to do a traditional resize
if possible before considering target hosts in another cell
which will be slower and more complicated. If the source cell
is disabled or target flavor is not available in the source cell,
then we'll have no choice but to select a host from another cell.
But all things being equal between hosts, we want to stay within
the source cell (by default). Therefore this change adds a new
CrossCellWeigher and related configuration option to prefer hosts
within the source cell when moving a server. The weigher is
completely noop unless a cross-cell move is permitted by
configuration, which will be provided in a future change.
Part of blueprint cross-cell-resize
Change-Id: Ib18752efa56cfeb860487fe6b26102bb4b1db038
18179aee4e