Fix gnt-cluster verify with diskless instances

Fix gnt-cluster verify with diskless instances

`gnt-cluster verify` was failing with KeyError if there was any
diskless instance in the cluster. This was because _CollectDiskInfo()
was not including these instances in the returned dictionary, but they
were expected to be present in LUVerifyCluster.Exec().

With this commit, we ensure that the dictionary returned by _CollectDiskInfo
includes entries for diskless instances as well.

Signed-off-by: Adeodato Simo <dato@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>