support --count on wsk <collection> list

[rabbah]

The API ~will soon~ now supports a count operation on collections. See https://github.com/apache/incubator-openwhisk/pull/3155.

[mdeuser]

@rabbah - is this example still a valid reflection of the updates

GET /namespaces/_/activations?count=true return {"activations": N}

[rabbah]

Yes.

[dubee]

@mdeuser, @drcariel, we should support count for wsk activation list.