Added elastic agent attributes and elements
Added elastic agent attributes and elements

* documentation for `<agentConfig/>` element on a `<job/>`
* documentation for `elasticAgentId` and `elasticPluginId` attributes of `<agent/>`

gocd/docs.go.cd,ketan/docs.go.cd,gocd/docs.go.cd,ketan/docs.go.cd,gocd/docs.go.cd,ketan/docs.go.cd