heat engine : watchrule quietly discard unused metric data

Some cfn-push-stats options try to send multiple metrics to a
watch, which won't work with our simplified CW implementation
which expects exactly one Metric associated with each watch
(since that's all you can define via the template)
So quietly discard any metric data we aren't interested in.

ref bug 1133510

Change-Id: I5159611b96c501ff2d8e6e605cbd3d4ae5f77e48