Change OpClusterVerifyDisks to per-group opcodes

Until now verifying disks, which is also used by the watcher,
would lock all nodes and instances. With this patch the opcode
is changed to operate on per nodegroup, requiring fewer locks.

Both “gnt-cluster” and “ganeti-watcher” are changed for the
new interface.

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