JCLOUDS-514: Add ability to attach block volumes at boot through the Nova ServerApi

Fixes: https://issues.apache.org/jira/browse/JCLOUDS-514
Not sure I agree with that. For me this is clearer as it tests the whole fea/prevents us from having to preserve info about the server between the tests and I think that the stack trace should be enough to indicate where the problem is. The other tests here require volume detachment as well.