LUClusterVerifyConfig verifies a number of configuration settings. For
doing so, it needs a consistent list of nodes, groups and instances. So
far no locks were acquired at all (except for the BGL in shared mode).
This is a race condition (e.g. if a node group is added in parallel) and
can be fixed by acquiring the BGL in exclusive mode. Since this LU
verifies the cluster-wide configuration, doing so instead of acquiring
individual locks is just.

Includes one typo fix and one docstring update.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>