With this change, LUClusterVerifyConfig becomes a "light" LU that only
verifies the global config and other, master-only settings, and the bulk of
node/instance verification is done by LUClusterVerifyGroup, which only acts
on nodes and instances of a given group.

To ensure that `gnt-cluster verify` continues to operate on the whole
cluster, the client creates an OpClusterVerifyGroup job per node group; for
convenience, the list of node groups is returned by LUClusterVerifyConfig.

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