add server_port to log format
add server_port to log format

based on feedback from @dancarley; $server_name is not necessarily
unique and may be empty or '_' for catch-all vhosts. However, a
combination of $server_name and $server_port *will* be unique on a
given source host.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet