Rename logname.generate to expander.expand

logname.generate was way to specific to its original use, despite its
highly generic functionality.
Add %a and %h to core string expander

%a is expanded to appName and %h to hostname, assuming those keys exist
on the worker object given to the expander.

Parameters that are not provided should be ignored and not substituted.
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