Fix bug #50 concerning 'rozo expand'

'rozo expand' raised an error when the parameter '-v <vid>' was set
This bug was supposed to be fixed but it still raised errors when no parameters
were set. #50 has been re-opened ...

* 'rozo expand' without parameter '-v' add nodes in new cluster & volume
* 'rozo expand' with '-v <vid>' add a cluster in the desired volume
Merge branch 'hotfix/1.2.5'