The recent work on multi-VG support has converted LUClusterVerifyDisks
into doing serialised calls to each node, as each node can have
different VGs. This is suboptimal, especially for big clusters, where
this LU is executed by the watcher very often.

This patch changes the logic based on the observation that querying a
node for its VGs and then requesting a LV list for those VGs is
equivalent to simply asking for all LVs, without specifying the VG
name(s). So backend.py needs changes to accept an empty VG list, and
the LU itself partially reverts to the previous version.

Additionally, we do two other fixes to this LU:

- small improvement in getting the instance list from the config
- MapLVsByNode works for all disk types, hence no need to restrict to
  the DRBD template, especially as today we can "recreate" disks for
  plain volumes too (the warning message in gnt-cluster is updated
  too)

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>