Fix generation of clusters via 'genEmptyCluster'

The current uniqueness algorithm (generate random node names, suffix
them with node index) is actually wrong: a node named "21" at index 5
will end up with the same name as a node named "2" at position 15.

To fix this, we also add a character from a different "set" ("-"), so
that such mixups can't happen again, and also add an explicit check
for it.

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