Added doc support for Agents API

Agents API extension was not documented in API Doc page
Modified:   src/docbkx/api-ref.xml > Added section to show Agents ref in api-site
New wadl:   src/wadls/compute-api/src/ext/os-agents.wadl > linking to samples in api_samples
Also modified existing json and xml samples for a clearer usage.
Fixes:  bug 1078010

Change-Id: I75db8ccc64c519c1c76d10cc59e20fc6c5fb8058
Merge "Added doc support for Agents API"