unit tests: Test dependency resolution using validate()

It doesn't really matter what triggers the error, we just want to check
that it happens during validation.

Change-Id: Ibfed738683973d35413b15bc9f173a51558c368c
Don't wrap HeatExceptions during validation

If an error of a known type (i.e. already a HeatException) occurs during
validation, don't wrap it in a StackValidationFailed exception.

Change-Id: I136a0def74c3edd5513f8acf6847428706ea2c06
unit tests: Check validation in VPC test

Checking for errors from creating a stack is not a representative test,
since in practice all stacks are validated before being created.

Change-Id: Ie6ef2c07648e4e9bd0375c56c99c8b03576fe7f8
unit tests: Don't mess with template retrospectively

In a couple of places we were modifying the template after creating the
stack with it, which is obviously not guaranteed to work.

Change-Id: I572cf6d7ac58392a9eb983ba8602481d0123c401
Merge "unit tests: Test dependency resolution using validate()"
Merge "Don't wrap HeatExceptions during validation"
Merge "unit tests: Check validation in VPC test"
Merge "unit tests: Don't mess with template retrospectively"