Fix fill_metadata usage for the ImagePropertiesWeigher

When using the weigher, we need to target the right cell context for the
existing instances in the host.
fill_metadata was also having an issue as we need to pass the dict value
from the updated dict by keying the instance uuid, not the whole dict of
updated instances.

Change-Id: I18260095ed263da4204f21de27f866568843804e
Closes-Bug: #2125935
Signed-off-by: Sylvain Bauza <sbauza@redhat.com>

doc/source/admin/scheduling.rst

@@ -1104,6 +1104,14 @@ provided in the option value.
 The resulted host weight would then be multiplied by the value of
 :oslo.config:option:`filter_scheduler.image_props_weight_multiplier`.
 
+
+.. note::
+
+    The weigher compares the values of the image properties as strings. If some
+    image properties are lists (eg. hw_numa_nodes), then if the values are
+    ordered differently, then the weigher will consider them as different
+    values.
+
 Utilization-aware scheduling
 ----------------------------