Implement support for Nova V2.1 API

Nova V3 API was deprecated in favor for Nova V2.1 API. since
we had it offered only optionally and disabled by default, and
since it is currently completely broken, lets just remove it
alltogether and enable V2.1 API instead, which is going to be
stable going forward.
Merge pull request #397 from dirkmueller/master

Implement support for Nova V2.1 API