Bring system tests back to "full" coverage by testing update().
Bring system tests back to "full" coverage by testing update().

We still don't test cluster.undelete() and don't explicitly
test cluster.delete() but it is used throughout (and a failure
would cause many tests to fail).

joerideturck/gcloud-python-bigtable,dhermes/gcloud-python-bigtable,dhermes/gcloud-python-bigtable,joerideturck/gcloud-python-bigtable