Add a test for customizing AWS::EC2::Instance

InstanceGroup explicitly supports "custom" versions of AWS::EC2::Instance
(instead of directly using heat.engine.resources.instance.Instance)
but there was no unit test for that feature. Here we add one.

Change-Id: I490675c18734a61d27625ed3c76fb228e3fd1a9d
Merge "Add a test for customizing AWS::EC2::Instance"