Improve error response in cfn Outputs validation

Relying on discussion in commit
I0d7e3e57e479ddb658645b8664bc847866180cda, this patch
improve error message for a better understanding of
error in Outputs and Resources sections.

Change-Id: I2a16093a57b474ef4636975b5fc0df4103678a08
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"
Merge "Improve error response in cfn Outputs validation"