crush: encode can override weights with weight set

crush: encode can override weights with weight set

Encode a "legacy" crushmap if (1) we're using weight sets, (2) the
client is lacking features for the weight set, but (3) the weight set
has a single position and no id remapping. Since these maps are only
used by clients (not humans), then there is no need to preserve the
original crush weights. We can just swap them for the real weights and
the legacy clients will behave as expected.

Fixes: http://tracker.ceph.com/issues/19836
Signed-off-by: Loic Dachary <loic@dachary.org>