Generate useful default scope for statsd metrics

Default is "%a.%h.%w" using the builtin string expander.
 * appName (%a) comes from strong-agent (agent.config.appName)
 * hostname (%h) comes from strong-agent (agnet.config.hostname)
 * workerId (%w) comes from strong-supervisor's clustering support

SLN-1186 #resolves