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
Set a small max_template_size for test_long_yaml

Currently this test takes 22 seconds to run. As the intent
of the test seems to be to assert that limits are enforced,
this change reduces the default limit to speed test execution time.

Change-Id: Ie12c53b832c993711c2187004d015b65045da901
Fixes-Bug: #1221478
Merge "Only validate credentials on create based on resources"
Merge "Set a small max_template_size for test_long_yaml"