Add Dimensions Default in AWS_CloudWatch_Alarm.yaml

Relying on [1], property Dimensions isn't required, but
this template has no Default value for Dimensions
(thereby making it required). This patch fix that omission.
Besides that, parameters have wrong parsing, because if
parameter with type 'CommaDelimitedList' has default
value '', parameter's parsing result would be [u''] instead
of []. this is wrong, so this patch fix it.

[1] http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-cw-alarm.html

Change-Id: I259249659c8b5dc846432f8e08985b148b30d682
Closes-bug: #1386824
Merge "Add Dimensions Default in AWS_CloudWatch_Alarm.yaml"