When we signal a resource, we want the signal to be interpreted in the
context of the properties of the resource as they were set at the last
stack update, not based on any attributes of other resources that may have
changed.
We already store the properties for comparison during updates. This patch
freezes the properties while we call handle_signal(), in a similar way to
how we freeze the properties during handle_update() (see bug 1584623).
Also make sure that when a ScalingPolicy adjusts an autoscaling group, it
also uses the group's stored properties so that e.g. the definitions of
existing members cannot change on a scale up.
Change-Id: I7e248ad82f2334b1a580655efa3a302e7d84fbd8
Partially-Implements: blueprint stack-definition
efd51f4525