Fix cluster verification issues on multi-group clusters

This patch attempts to fix a number of issues with “gnt-cluster verify”
in presence of multiple node groups and DRBD8 instances split over nodes
in more than one group.

- Look up instances in a group only by their primary node (otherwise
  split instances would be considered when verifying any of their node's
  groups)
- When gathering additional nodes for LV checks, just compare instance's
  node's groups with the currently verified group instead of comparing
  against the primary node's group
- Exclude nodes in other groups when calculating N+1 errors and checking
  logical volumes

Not directly related, but a small error text is also clarified.

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