cli.py: use None as name for tag operations on the cluster

This change is mostly cosmetic. Previously, the literal "cluster" was
used for the 'name' field of tag operations on the cluster (as opposed
to a node or an instance). Since this field has a type of TMaybeString
specifically for the case of the cluster, it seems more correct to use
None, rather than an arbitrary string (that is not used by the callee).

Additionally: note in opcodes.py that groups also expect a name; the
previous comment only referred to nodes and instances.

Signed-off-by: Dato Simó <dato@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>