LaunchConfiguration replace on changed Metadata

This makes AWS::AutoScaling::LaunchConfiguration raise
an UpdateReplace if its template Metadata is changed on a
stack update. Any changes to LaunchConfiguration should result
in the resource being replaced, since this is how those changes
propagate to the scaling group.

This fixes a regression which has likely existed since
update_allowed_keys was removed: I78494aba07b6badeca8e95f1875ca8e4b9b0290c

Change-Id: I3a53422ed862777dffb9cac28088fbfe8dd417fa
Closes-Bug: #1287953
Merge "LaunchConfiguration replace on changed Metadata"