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

Fixes: https://issues.apache.org/jira/browse/JCLOUDS-514
> most of the Nova Live Extension API tests follow this pattern

Lazy me...I was only looking at the diff ;-) Fine to keep as-is, and thanks for explaining, @jasdeep-hundal and @jdaggett!