service::node: Allow users to specify the logging name
Some services use the same service::node definition in different
contexts, so allow them to specify different logging names and statsd
prefixes for them.
One such case is Parsoid. In production, the 'parsoid' tag is used, but
on ruthenium (acting as the round-trip host), the logs and metrics ought
to have a different name. This patch also sets them to 'parsoid-tests'