validate-template is not as strict as pre-flight validation

[Zane Bitter]

In CloudFormation, ValidateTemplate gets only the template and not any of the parameter values. (This is because it's used to generate the UI for requesting parameter values from the user.)

In the OpenStack API, that's not necessarily the case since the fix for bug 1298450. The user can supply the parameter values, but we don't use them. Specifically, we set two flags in validate-template that we do not set during pre-flight validation of a create or update:

* strict_validate=False on the Stack - this prevents us from examining any Property values. We validate only which properties are specified. (This predates the ability to pass parameter values to validate-template - see bug 1154722.)
* validate_res_tmpl_only=True argument to Stack.validate() - this prevents Resource plugins from supplying their own custom validation. See bug 1467573.

We need to maintain backwards compatibility with the case where parameter values are not supplied (bug 1467573 was in fact about fixing where we didn't), since the output is used to generate a UI in some cases. But if parameter values *are* supplied, then we ought to be able to validate as much as the pre-flight checks on create and update can without the user necessarily having to create a stack to check it.