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>