cli/zip: Use "cluster" to identify the logical cluster instead of "te…

…nant"

Release note (cluster virtualization): This patch renames the
subdirectory inside the output of `cockroach debug zip` when pointed
to a cluster with virtualization enabled to be `cluster`, which brings
the UX in coherence with the parameter of the same name in `cockroach
sql` and the HTTP API parameter in the same name.