The cucumber tests for ec2 and mock drivers had a few bugs as follows:
* 'api_steps.rb' doesn't check xml attributes for hardware_profiles and
instances. In some situations, this hangs the cucumber process (bug in xpath
library?).
* some features attempt to check xml attributes (e.g., each 'image' should have
'architecture' attribute set to 'i386'),
but there is no such attributes (architecture) under the target element
(image). So I added a new feature and the corresponding step definition which
checks child elements instead of attributes.
