Only validate credentials on create based on resources

Only validate credentials on create based on resources

This change relaxes the validation which checked for credentials
on stack create and update.

As implemented, having any of the following resources
in the template will result in credentials being mandatory
on create and update:
* AWS::AutoScaling::ScalingPolicy
* OS::Heat::HARestarter
* AWS::CloudFormation::WaitConditionHandle

For all other templates, credentials are not needed.

When trusts are merged, this logic could also be used to decide
whether a trust token needs to be created at all.

Fixes bug: #1217617

Change-Id: I3e4b8698d3712053dc3c0851433ef0cbbadbdfed