sensors at riak cluster level and other minor tidies for riak and couchbase

[ahgittin]

@bostko hi can you review this?

[bostko]

Very good changes, thank you!
I tested the cluster avarage values with https://github.com/apache/incubator-brooklyn/blob/master/examples/simple-web-cluster/src/main/resources/brooklyn/demo/nodejs-riak-todo.yaml and it looks good

P.S. I think it would be better if changes in doStart and those for cluster stats were in separate commits

[asfbot]

incubator-brooklyn-pull-requests #1361 SUCCESS
This pull request looks good

[aledsage]

Would be good if the Riak cluster tests were updated to assert that the NODE_PUTS_1MIN_PER_NODE etc are being populated (e.g. that the attribute is eventually non-null).

[aledsage]

Looks good; would be nice to improve our test coverage for things like this (even if it is just that the attribute values are non-null). Merging this now.